Buy-out giants Kohlberg Kravis Roberts & Co (KKR), Permira and Apax Partners have emerged as the most lucrative clients in the sector in recent years, according to new research.

Mergermarket rankings compiled exclusively for Legal Week show the private equity houses have completed the highest value of deals in Europe over the last three years.

The news will be of particular interest to Freshfields Bruckhaus Deringer, Clifford Chance (CC) and Ashurst, which enjoy strong ties to the buy-out houses.

KKR comfortably outstripped the competition, leading the line-up with 14 European M&A deals worth a total of A66.3bn (£46.1bn). The client is best known for using US firm Simpson Thacher & Bartlett and CC.

Much of KKR's strong performance can be attributed to its £9.7bn bid for Alliance Boots at the beginning of the year, which generated lead roles for both advisers.

Permira, a key client of both CC and Freshfields, finished in second spot in the line-up with 23 deals at A39.9bn (£27.7bn) while Apax, which historically instructs Ashurst and Freshfields, took third place after handling 34 deals in the three-year period worth A37.6bn (£26.1bn).

The rankings see many of the UK's most prominent buy-out houses usurped in the European market by a string of US and international investors including Blackstone Group, GS Capital Partners and Providence Equity Partners, which all finished in the top 10.

Some pension funds also make an appearance, with Singaporean government group GIC Special Investments coming sixth and Canada's Caisse de Depot et Placement du Quebec coming eighth.

Australia's Macquarie Bank came in fifth place with a tally of 19 deals worth h29.1bn (£20.2bn).

Mid-market outfit 3i Group completed the greatest number of deals, working on 81 transactions worth h17.9bn (£12.4bn) – a long way ahead of its closest rival, Barclays Private Equity, which amassed 51 deals to the value of h5.1bn (£3.5bn).