On September 3, 2026, the UK Competition and Markets Authority (CMA) published revised Merger Assessment Guidelines (MAGs). The revised MAGs replace the previous guidelines from March 18, 2021.[1]

The only substantive change is greater discussion of merger efficiencies. While the revised MAGs retain the underlying analytical framework for efficiencies, they provide welcome additional detail on when efficiencies can arise, what evidence the CMA will consider, and how merging parties should engage with the CMA.

Link to The Framework for Efficiencies is Unchanged The Framework for Efficiencies is Unchanged

The revised MAGs do not fundamentally change the CMA’s framework for assessing efficiencies. The MAGs retain the distinction between (i) rivalry-enhancing efficiencies (which can prevent a substantial lessening of competition (SLC)); and (ii) relevant customer benefits (which do not prevent an SLC, but may outweigh any adverse effects).[2]

Rivalry-enhancing efficiencies must:

  • enhance rivalry in the supply of products in the market where an SLC may otherwise arise;
  • be timely, likely and sufficient to prevent an SLC from arising;
  • be merger-specific; and
  • benefit customers in the UK.[3]

Relevant customer benefits must also be merger-specific and benefit customers in the UK. In addition, they:

  • Must provide a benefit to relevant customers through lower prices, higher quality or greater choice of goods or services, or greater innovation in relation to such goods or services.
  • Can relate to any market in the UK (not limited to the market or markets in which the SLC has occurred).[4]

In practice, the CMA has rarely accepted efficiencies arguments. The CMA has rejected rivalry-enhancing efficiency arguments notwithstanding submissions on cost savings, integration benefits and improved R&D capabilities, typically on the basis that the parties had not provided sufficient evidence that the claimed efficiencies were merger-specific, timely, likely or sufficient to prevent an SLC.[5] Until recently, no merger had been cleared on the basis that rivalry-enhancing efficiencies offset the anti-competitive effects of the merger, preventing an SLC from arising.

The CMA’s clearance of Vodafone/Three on December 5, 2024 was the first significant departure from this pattern.[6] The CMA accepted that the merger could generate efficiencies, including accelerated deployment of next-generation 5G networks through an £11 billion investment program, but found that the merged entity would lack the incentive to deliver that investment in full without a binding commitment. The CMA therefore required legally binding remedies to secure the efficiencies and it was on this basis that the case was ultimately cleared.[7]

Link to The Revised MAGs Further Clarify The CMA’s Approach to Applying the Existing Framework The Revised MAGs Further Clarify The CMA’s Approach to Applying the Existing Framework

While the new MAGs do not depart from the established framework, they do provide additional detail on how the CMA expects to apply it in practice. For example:

  • The CMA will only engage with efficiency claims which are supported by ‘verifiable’ evidence. The MAGs note that synergies often form part of parties’ rationale for a merger, and acknowledge that studies have found that these can give rise to benefits for consumers. That said, the MAGs cite certain studies which indicate efficiencies “may not be realised and/or are not passed through sufficiently to offset the loss of competition”.[8] The MAGs state that, in light of this, the CMA will take a case-by-case approach to assessing efficiency claims and will only take these into account “when they are supported by verifiable evidence” relating to the nature and extent of the efficiencies in question, as well as firms’ incentives to realize them and pass on the benefits to customers.[9]
  • Contemporaneous internal documents will be given higher evidential weight, although there is some scope for ‘bespoke’ analysis. In assessing the evidence relating to the benefits from a merger, the revised MAGs state that the CMA is likely to place greater weight on materials that were generated in the ordinary course of business, for example: (i) operational and financial data; (ii) strategy and merger rationale documents; (iii) transaction materials; and (iv) the merger firms’ track record in realizing and passing-on benefits in previous mergers. In relation to internal documents specifically, the revised MAGs state that the CMA will consider the context, purpose and effect of the internal document (e.g., whether it was a document for the board or executive decision-making) and its consistency with other evidence, in considering the document’s probative value. In addition, the MAGs state that the CMA will consider bespoke analysis provided by merging parties where internal documents “may not address all aspects of the CMA’s efficiencies framework”. The MAGs give the example of evidence relating to the merging parties’ ability and incentive to innovate, or to pass through benefits to customers.[10]
  • There is scope for non-price and longer-term benefits to be taken into account. The revised MAGs state that quantitative evidence on how customers value non-price improvements, such as survey data or spending patterns, may be used to help establish the likely scale of a benefit even where the underlying efficiency cannot be precisely quantified.[11] The MAGs also state that the CMA will take account of market-specific investment and innovation cycles in assessing whether efficiencies arise on a sufficiently timely basis, acknowledging that certain dynamic efficiencies (i.e. firms’ ability to improve productivity, lower costs and innovate) may not be fully realized for several years.[12]
  • Uncertainty about the outcome of investment and innovation efforts is not a bar to a successful claim. The MAGs recognize that there may be some uncertainty about the outcome of investments and innovation efforts, including about whether these will ultimately result in products or services being made available to customers, the timeframe within which this may occur, and/or the scale of the resulting benefits. In such cases, the CMA may consider whether the merger is likely to strengthen the process of dynamic competition.[13] This is consistent with the CMA’s approach to uncertainty when considering the impact of a merger on dynamic competition as part of its assessment of potential anti-competitive effects.[14]
  • The CMA will consider whether claimed efficiencies could be achieved through alternative means to the merger. The MAGs retain the requirement for efficiencies to be merger-specific. The MAGs state that the CMA will assess whether a claimed efficiency could realistically have been achieved without the merger. For example, the CMA will consider the feasibility of alternatives and whether pursuing them would have been commercially rational for the merging parties.[15]
  • Parties should engage early. The MAGs urge merging parties to engage with the CMA early on efficiencies, to provide greater scope for feedback and to provide sufficient time for the CMA to assess the parties’ arguments and, if needed, test these with third parties.[16]

Link to Key Takeaways Key Takeaways

  • Efficiency claims must be supported by verifiable evidence, ideally ordinary course documents. The MAGs indication that the CMA will place greater weight on materials generated in the ordinary course of business means merging parties should ensure that internal strategy papers, board materials and financial models consistently reflect the efficiency rationale for a transaction. Although there is scope to supplement this evidence with ‘bespoke’ submissions, the MAGs make clear that the CMA’s primary focus will be contemporaneous documentation.
  • There is greater scope to put forward non-price and dynamic benefits. This is particularly relevant in innovation-driven sectors, though the CMA will continue to test the likelihood and timeliness of such claims rigorously.
  • Engage with the CMA early. Waiting until later in a review to introduce evidence on efficiencies is likely to reduce the weight the CMA is prepared to give it.

[1] CMA, Merger Assessment Guidelines – GOV.UK.

[2] CMA, Merger Assessment Guidelines, paragraph 8.5.

[3] CMA, Merger Assessment Guidelines, paragraph 8.14.

[4] CMA, Merger Assessment Guidelines, paragraph 8.36.

[5] See, for example, Intercontinental Exchange (ICE)/Trayport, CMA Final Report (17 October 2016); Thermo Fisher Scientific/Roper Technologies, CMA Provisional Findings (17 April 2019); Tobii/Smartbox, Final Report (15 August 2019); and Microsoft/Activision, CMA Final Report (26 April 2023).

[6] SeeCMA, Vodafone/CK Hutchsion JV merger inquiry case page (available at: https://www.gov.uk/cma-cases/vodafone-slash-ck-hutchison-jv-merger-inquiry).

[7] See Cleary Antitrust Watch, Learnings from Vodafone/Three.

[8] CMA, Merger Assessment Guidelines, paragraph 8.11.

[9] CMA, Merger Assessment Guidelines, paragraph 8.12.

[10] CMA, Merger Assessment Guidelines, paragraph 8.9.

[11] CMA, Merger Assessment Guidelines, paragraph 8.29.

[12] CMA, Merger Assessment Guidelines, paragraph 8.22.

[13] CMA, Merger Assessment Guidelines, paragraph 8.30.

[14] CMA, Merger Assessment Guidelines, paragraph 5.20.

[15] CMA, Merger Assessment Guidelines, paragraphs 8.32 to 8.34.

[16] CMA, Merger Assessment Guidelines, paragraph 8.41.