Ashurst US energy partner Charles Williams has left to launch a boutique, with his exit coming as the UK firm also hires a partner into the practice in New York.

Williams, who joined Ashurst's New York office from Allen & Overy (A&O) in 2013, will be joined in his new venture by three partners, all of whom have previously worked at US firms including Latham & Watkins, Mayer Brown and Wilmer Cutler Pickering Hale and Dorr.

The boutique will have offices in New York, Washington DC and Los Angeles.

Before becoming one of a growing string of exits from Ashurst when he left earlier this month, Williams was a partner in the firm's energy, transportation and resources practice. His practice focuses on the development and project financing of energy and infrastructure projects, public-private partnerships, and private equity investment in the energy and infrastructure sectors.

Prior to his move to Ashurst, Williams spent almost 14 years at A&O in the US, making partner in 2007.

Williams' departure coincides with Ashurst investing in its US infrastructure offering with the hire of Vincent Casey from Nixon Peabody as a partner.

Casey had been a partner in the US firm's infrastructure group since 2014, and previously worked at both Shearman & Sterling and Orrick Herrington & Sutcliffe in the US.

He is the second partner to join Ashurst's New York infrastructure group in recent months, with the appointment following that of Andrew Fraiser from Allen & Overy to lead the team in July.

Frasier said: "Ten years ago, western Europe was the hub for global capital flow into infrastructure, but we are now seeing that capital move further afield, most notably into the Americas and the Asia-Pacific region. More and more infrastructure developers and investors are moving into the US and they are placing a high value on knowledge to assist them. We are looking to harness those opportunities."

Ashurst global head of infrastructure Mark Elsey added: "The US is probably the biggest infrastructure market in the world and offers increasing opportunities.  We are starting to build up a team that we think is best suited to exploit these opportunities."

The firm currently has 14 lawyers in its US infrastructure practice and Elsey said that by the beginning of the next financial year, he would like to grow the core US team to at least 20.

He added: "At the moment we are consolidating our position. We are looking at our abilities to grow in New York through leveraging off our practice and broader global platform, with particular focus on Latin America and other outbound financing work from the US."

Ashurst launched in New York and Washington, DC in 2009, when an 11-partner team arrived from McKee Nelson. Its Washington office has five lawyers, including three partners.

According to its website, Ashurst currently has 36 lawyers in its New York office, including 14 partners following Casey's arrival.

Williams' departure represents the 13th partner exit from Ashurst to emerge this month. Four office heads have departed in recent weeks, including Brussels managing partner Carl Meyntjens and Singapore managing partner Shaun Lascelles. Former Singapore head Keith McGuire had handed in his notice in June.

Earlier this summer, Ashurst posted disappointing financial results for 2015-16, with profit per equity partner falling back to a more than 10-year low of £603,000 after two consecutive years of decline, while revenue fell by 10%.

Other Ashurst partner exits to emerge so far in 2016

September

Brussels managing partner Carl Meyntjens – Simmons & Simmons Singapore managing partner Shaun Lascelles – understood to be joining Vinson & Elkins London banking partner Nigel Ward – Paul Hastings Hong Kong managing partner Lina Lee – A&O Hong Kong capital markets partner Jonathan Hsui – A&O Hong Kong restructuring partner Bertie Mehigan – Howse Williams Bowers Abu Dhabi managing partner Alastair Holland – Curtis Mallet-Prevost Colt & Mosle London finance partner Michael Smith – Paul Hastings London finance partner Diala Minott – Paul Hastings London finance partner Cameron Saylor – Paul Hastings London regulatory partner Nicola Higgs – Latham & Watkins London energy partner Nicolas Bonnefoy – now managing director at Africa Oil & Gas

August

Hong Kong finance partner Doo-Soon Choi – Mayer Brown JSM

July

London global co-head of financial regulation Rob Moulton – Latham & Watkins London restructuring and insolvency partner Simon Baskerville – Latham & Watkins London restructuring and insolvency partner Diane Roberts – Reed Smith London chief financial officer Brian Dunlop Frankfurt finance partner Tom Beckerhoff – Noerr

June

Singapore managing partner Keith McGuire – PwC Legal Stockholm office managing partner Jon Ericson – Hamilton Stockholm corporate and real estate head Pontus Bergsten – Hamilton Stockholm finance partner Eric Halvarsson – Hamilton Stockholm competition partner Mats Johnsson – Hamilton

May

Frankfurt tax partner Heiko Penndorf – Goodwin London disputes head Mark Clarke – White & Case

April

London financial regulation partner James Perry – Gibson Dunn & Crutcher London capital markets partner Jonathan Parry – White & Case London derivatives and structured products partner Christopher Georgiou – director at Apollo Consultants London labour and insolvency partner Andreas Vogel – consultant London corporate partner Anthony Clare – Stephenson Harwood