The largest US law firms in London added some 500 lawyers to their City ranks last year, making more than 100 lateral partner hires as the weaker pound and the King & Wood Mallesons (KWM) collapse helped the group of firms ramp up their presence on this side of the Atlantic.

Between them, the 50 largest US firms with UK law practices in the City added 109 new partners in London during 2017, up marginally on the previous year.

Legal Week's research – which excludes transatlantic vereins DLA Piper, Norton Rose Fulbright and Hogan Lovells – shows that, between them, the group of firms now house more than 1,500 partners and almost 3,500 further lawyers.

Four US firms – White & Case, Baker McKenzie, Reed Smith and Latham & Watkins – each have more than 300 lawyers in the capital, with a further two – Kirkland & Ellis and Jones Day – housing at least 200 apiece.

Three firms – Reed Smith, White & Case and Bakers – had at least 100 partners when the data was collated on 1 January this year, with Kirkland, Mayer Brown and Latham not far behind on at least 74 partners each.

As a comparison, City stalwarts Macfarlanes and Travers Smith both had fewer than 90 partners during the last UK financial year, while no more than 10 partners separate Reed Smith and White & Case's London partner headcounts from Slaughter and May's figure of 115 last year.

But while the sheer scale some of these firms have now achieved in London may make for depressing reading for some UK rivals, the research shows much of their partner-level hiring was driven by the collapse of KWM's European arm last January.

Four of the US firms making the most London lateral hires during the 2017 calendar year – Reed Smith (13), Goodwin Procter (nine), Greenberg Traurig (seven) and Covington & Burling (six) – all benefited from the implosion of the legacy SJ Berwin business, adding large teams of KWM lawyers as they found new homes.

Seven of Goodwin's nine lateral partner hires in 2017 joined from KWM, including funds partner Michael Halford, with this recruitment helping the US firm to more than double its lawyer count in the UK capital from 37 to 77.

Goodwin London co-chair Samantha Lake Coghlan commented: "We have had a great 12 months of rapid growth in London, both in terms of the addition of senior new hires and in terms of growing our existing key franchises in real estate and private equity.

"The year has also seen us add talent in technology and life science, new areas of focus for us in Europe, but which are practice areas where we have a leading position in the US. The European expansion underlines our commitment to be best in class across four major business verticals. We expect this rapid growth to continue this year and beyond."

Similarly, Greenberg achieved the second highest year-on-year growth thanks in part to six partners and a number of other lawyers joining from KWM to increase London lawyer count by nearly 66% to 76.

Executive chairman Richard Rosenbaum said: "The London market is key to Greenberg's global platform. The level of talent and clients in the market allows us to continue our path of strategic growth, without resorting to mergers or vereins, and the results speak for themselves.

"I am particularly impressed by how well [former name partner] Paul Maher has adapted. Last year, he took his name off the door some years after founding the office, and while continuing to work closely with me on London strategy, became a vice-chairman of the global firm."

In total, partners from the failed KWM business accounted for some 35 of all of the lateral hires in London by the group of US firms, equating to just under a third of all partner-to-partner level moves across the group.

Leaving KWM to one side, there were a number of high-profile magic circle hires by US firms, including Patrick Sarch (pictured) moving from Clifford Chance to White & Case, Ed Barnett's switch from Allen & Overy to Latham, and Ben Spiers leaving Freshfields for Simpson Thacher.

However, many of the year's standout lateral hires were between US rivals, with hunters just as likely to find themselves the prey.

Roughly a quarter of all of last year's hires (27) were between rival US firms. Standout examples in this group include private equity partner Richard Youle's move from White & Case to Skadden Arps Slate Meagher & Flom, while Latham brought in two partners from Quinn Emanuel.

With US firms including Ropes & Gray and Mayer Brown among those seeing partner exits to rivals, a number of firms saw their London lawyer count shrink on last year.

Ropes, for example, saw lawyer count drop 18% from 129 to 106 during 2017, after exits including restructuring partner James Douglas to Linklaters and finance duo Mark Wesseldine and Fergus Wheeler to King & Spalding. Other firms to see lawyer count reduce year on year included Arnold & Porter, Simpson Thacher & Bartlett, Mayer Brown and Shearman & Sterling – all of which saw declines of between 9% and 12%.

Latham and Kirkland, however, continued to set themselves apart from many of their rivals, despite Kirkland seeing a number of partner exits of its own.

Both saw double-digit increases in City lawyer count during 2017, and both now have London revenues way ahead of many UK top 50 firms. Kirkland, the smaller of the two in London with 217 lawyers, compared with Latham's 335, is understood to have made about 10% of its recordbreaking 2017 global revenue figure in London – equating to about $315m (£233m).

Commenting on last year's lateral recruitment market, Vassos Georgiadis, founder and managing director of Melton Legal, said: "US firms continued very strong growth in London in 2017. The collapse of KWM was the single largest event fuelling this, but across the board the first and second division UK firms continued to lose partners to the US firms, with very few coming the other direction.

"We expect 2018-19 to see more moves to US firms, notwithstanding steps from some of the principal UK firms to revamp their remuneration systems to prevent star partners leaving for financial reasons."

Fox Rodney Search managing director Leanne Clark added: "A lot of US firms are looking for the same expertise across similar practices. Leveraged finance and private equity are still really hot. US firms are getting a lot of coverage in the press for high-profile cases, such as Gibson Dunn & Crutcher's role on the competition side of the Asda-Sainsbury's merger. Winning roles on big-ticket work like this demonstrates their credibility in the UK market and attracts attention from partners looking to move.

"The continued trend of magic circle lateral moves to leading US firms is interesting – as a number of the magic circle firms change their lockstep culture and increase performance management, driven partners are less likely to see the move to a more merit-based US firm as such a significant change."

Top five fastest-growing US firms in London in 2017

• Goodwin – up 108.1% from 37 lawyers to 77
• Greenberg Traurig – up 65.2% from 46 lawyers to 76
• Kirkland & Ellis – up 28.4% from 169 lawyers to 217
• Quinn Emanuel – up 27.5% from 51 lawyers to 65
• Winston & Strawn – up 24% from 25 lawyers to 31

Top five US firms in London by lateral hiring activity in 2017

• Reed Smith – 13
• Goodwin – 9
• Greenberg Traurig – 7
• Covington & Burling – 6
• Latham & Watkins – 6

Top five US firms by London lawyer count at January 2018

• Baker McKenzie – 384
• White & Case – 370
• Latham & Watkins – 335
• Reed Smith – 317
• Kirkland & Ellis – 217