Clients increasingly struggle to draw a distinction between their US law firm advisers in London and their UK counterparts, so well established are many of the American firms in the City now.

And an analysis of the Limited Liability Partnership (LLP) accounts of a group of the City's biggest international imports paints a similar picture of strength, with many of the firms showing no great financial dependence on their overseas parents.

Data compiled for Legal Week by accountants Smith & Williamson from the UK LLP accounts of 17 US and international firms demonstrates how far these firms have developed in recent years.

The accounts – which are the latest available and cover a financial year ending between October 2014 and May 2015 depending on the firm – show six of the 17 firms posted revenue of more than £100m. This figure would put them comfortably into the UK top 40 by revenue based on fee income generated in their UK LLP alone.

A further two firms – Dechert and Paul Hastings – would have made it into the UK top 50 based on the UK revenue figures.

The research includes King & Wood Mallesons' UK arm – legacy SJ Berwin – which unsurprisingly posted the largest turnover, with the UK LLP generating fee income of £190.3m in the year to 30 April 2015. Prior to SJ Berwin's 2013 merger with KWM, the UK firm reported 2012-13 revenue of £184.6m. Equally unsurprisingly given the history of its legacy UK business Dentons is the next largest, with revenue for the UKMEA LLP standing at £157m.

But it is not just those which have grown through mergers that are succeeding in London. White & Case's UK arm is only marginally smaller that Dentons', generating £154.4m in 2014, with Reed Smith and Baker & McKenzie not too far behind. Bakers, whose accounts relate to the year to 30 June 2014, estimates that its 2014-15 LLP results will show revenue of £140m. At the other end of the scale is Winston & Strawn's London practice, which brought in just £8.3m last year.

International firms in London – 2014-15 LLP accounts

Although largely performing well over the period (average revenue growth across the group was 5.9%), some of the firms failed to make year-on-year revenue increases, with Greenberg Traurig Maher, Orrick Herrington & Sutcliffe, Dechert, Winston and O'Melveny & Myers all seeing a drop in money brought in. Winston saw the largest decline at 10.1% down.

The strongest performer was relative newcomer Quinn Emanuel Urquhart & Sullivan, which continued to make sizable gains, growing its practice by almost a third in a year. K&L Gates and White & Case in London also saw double digit turnover improvements.

White & Case London head Oliver Brettle said: "We've more than held our revenue and PEP at a high level, increasing from a record year in 2014. London was among our top performing offices alongside New York, Paris and Washington."

Profitability
Looking at profitability, the group had, on average, a higher profit margin than firms in the UK top 50, although with a greater range. With an operating profit margin of 68.5% and an increase in operating profit of 36%, Quinn was the most profitable outfit in the group last year but firms such as McDermott Will & Emery, King & Spalding and Dechert all had margins in excess of 40%, despite most of the group seeing profit margin fall year-on-year.

Winston and O'Melveny posted the biggest drops in operating profit, with the former seeing its City operating profit fall by 88.5% and the latter posting a 45% decline.

London was among our top performing offices alongside New York, Paris and Washington – Oliver Brettle, White & Case

Quinn and Winston also sit at either end of the spectrum when measuring operating profit per member. This stood at £1.55m for each of Quinn's 15 members, compared with £13.7k at Winston, which has 11 members. This represents an 88% drop on last year's profit per member figure for Winston, reflecting the similar drop in operating profit.

Peter Crowther, managing partner, Winston & Strawn London, said: "It is difficult to compare like for like as it's Winston & Strawn's first year as an LLP. However, it's a year when we made significant investments including expanded office space and a number of prominent hires. We have a further programme of investment planned for the London office."

Headcount
With large variation in office size – from 877 lawyers and support staff at KWM to 38 at Winston – there is also a significant disparity in the fee income generated per fee earner. At Quinn, this stood at £1.26m per lawyer, while Dentons' lawyers brought in £385,000 on average. Even removing Quinn from the picture, fee earners at these firms bring in more than their counterparts at UK firms with similar revenues, with the average revenue per lawyer across the group standing at £571,400.

Looking at the group as a whole, lawyer and support staff numbers grew by 4.3% on average against a 1.4% increase in the number of members.

It's a year when we made significant investments including expanded office space and a number of prominent hires – Peter Crowther, Winston & Strawn

International and US firms also spent less on their lawyer and support staff as a proportion of revenue than the average UK Top 50 firm. Average staff costs stood at £25.1m, ranging from £75.8m at Dentons down to £3.5m at Winston and Greenberg.

With the exception of KWM – which has a £20m bank loan – Mayer Brown and Dentons, none of the firms hold any bank debt, in contrast to many of their UK peers.

Giles Murphy, partner and head of assurance and business services at Smith & Williamson comments: "Trying to set out a separate class of American law firms doesn't work so well these days. They are not quite so much a distinct category and are increasingly well integrated into the London, UK and European legal market and have become part of the mainstay. As firms get more established in the UK they are no longer treated like a 53rd state and partners tend to be paid in Sterling, acting for UK/international clients rather than being supported by the US."

He adds: "The big, big issue for US firms is the role the UK has in the European environment – and this is something the UK firms should be worried about too. US firms probably see London as a bridgehead into the rest of Europe so where the UK is at the moment must create some uncertainty. If I was sitting in the US getting requests from London to grow I would be thinking should I put the funding there or actually would somewhere like Paris or Germany be a consideration."

International firms in London 2014-15 LLP accounts