Clifford Chance (CC) corporate heavyweight Adam Signy has quit the City firm to join Simpson Thacher & Bartlett.

The move is one of the most high-profile partner appointments by a US firm in London for years and will be seen as highly significant.

Simpson Thacher currently has no UK-qualified corporate or private equity partners in London, despite links in the US with names such as Kohlberg Kravis Roberts & Co and Carlyle Group.

Signy (pictured right) has been a partner with CC since 1987 and has close ties with private equity houses including Candover as well as wide-ranging experience advising on public M&A deals. In 2007, when CC restructured its corporate practice he took control of the public M&A group alongside Matthew Layton, who has since been named as the firm's corporate head.

It is believed that Signy had held informal discussions with a number of other US firms in recent months before deciding to move to Simpson Thacher.

His departure will be seen as a blow for CC as, alongside corporate head Matthew Layton, he is one of the firm's biggest corporate names. In 2004 CC narrowly managed to hold onto both Layton and fellow private equity heavyweight James Baird when the pair entered into negotiations with Weil Gotshal & Manges.

One corporate partner at a magic circle rival said: "This is not the first significant corporate lawyer CC has lost and it raises questions about who they have now to credibly pitch to listed companies."

A City-based private equity partner said: "This is a resounding vote of no confidence for CC – but they have some excellent young corporate partners coming through."

Signy's hire will take Simpson Thacher's London office, which opened in 1978 and is headed by Greg Conway, to around 12 partners, of which the majority are US-qualified.

The New York leader's other high-profile UK hires have been on the finance side and include acquisition finance partners Tony Keal and Euan Gorrie, who both joined from Allen & Overy, and structured finance partner Stephen Short.

Some UK partners have raised questions about how easy it will be for Signy to make his mark at Simpson Thacher without a large team and backup from practice areas including tax.

One partner at a rival firm commented: "I am not sure what sort of platform there will be for Adam and he certainly will not be doing the sort of deals he has been used to at CC. I cannot see him drafting documents dayin, day out."

Commenting on the departure, Layton said in a statement: "Adam has made a major contribution to our corporate practice and we wish him well for the future."

For more analysis, see Editor's Blog: Signy quits – not an earthquake but a definite fault line

Sign up for legalweek.com job alerts