The debate over legal adviser rankings is set to be reignited as the latest quarterly M&A stats show a slew of US firms – many with limited or no local law practices – surging up the European deal league.

Skadden Arps Slate Meagher & Flom topped the latest Mergermarket league tables for European M&A in 2006, having advised on 36 deals worth €189.6bn (£130bn). Clifford Chance (CC) came second, advising on 125 deals worth €173.3bn (£119bn) in the year to date, just ahead of Linklaters.

In total, eight US firms, including Davis Polk & Wardwell and Sullivan & Cromwell, also featured in the top 20 league table for European deals by value.

A growing number of lawyers argue that shifts in the M&A market are making it harder for data providers to accurately reflect the performance of Europe's top deal firms at a time when big-ticket transactions are generating a slew of secondary roles. For example, 22 law firms were credited as advisers on Mittal Steel's €25.8bn (£17.7bn) merger with Arcelor.

In comparison a breakdown of Europe's top 25 deals of the year to date shows Freshfields Bruckhaus Deringer in pole position in value terms, taking six lead advisory roles with a combined total of €95.7bn (£66.1bn).

By the same measure of lead adviser roles, Skadden falls to fifth place in value terms.

Travers Smith head of corporate Chris Hale commented: "Sometimes you see firms that you would not associate with a particular field featuring high up because they have played a minor role in a major deal. The M&A tables are a good guide – but they need to be used judiciously."

Previous research by Legal Week and Mergermarket which focused on lead corporate roles on Europe's 200 largest M&A deals in 2005, found that Freshfields and Slaughters and May's performance are substantially understated by traditional league tables.

Slaughters corporate chief Chris Saul told Legal Week: "Tables are sometimes misleading because firms acting on one big deal are over-represented. But we all like to criticise their bad points, and we still like it when we come out top."

The rankings come as cross-border deal activity continues to rise. European M&A activity has increased 21% compared to the same quarter last year.