The resignation of Charlie Geffen from Ashurst this week is not a shock. His defeat to Ben Tidswell in last October's chairman election paved the way for intense year-long speculation on his next move. Throughout, the assumption both at Ashurst and in the wider market, was that remaining at the firm was not an option for the former senior partner.

The decision to move to the London office of Gibson Dunn & Crutcher, along with fellow private equity star Mark Sperotto, may be a bit more of a surprise. Given his profile, and the fact he had spent five of the last six years in a full-time management role, many would have expected Geffen to move in-house, or to a role with one of his clients. Offers will not have been in short supply.

But until now – while boasting one of the leading private equity practices in the US – Gibson Dunn has not made the same splash in the City as leading transatlantic buyout teams at Simpson Thacher, Weil Gotshal & Manges and Kirkland & Ellis.

What is clear from these hires is Gibson Dunn has huge aspirations for its City corporate practice. Before the hire of former Ashurst partners Jonathan Earle and Nigel Stacey earlier this year, it had set itself a target of around ten corporate partners in London. The office is now in advanced talks with another major name in private equity, whose hire had originally been set to run in conjunction with this week's appointments.

According to Gibson Dunn chairman and managing partner Ken Doran, who has known Geffen for a number of years, the latest recruits show the firm is assembling "one of the strongest, competitively dynamic teams in the City".

Another senior private equity partner, who said he had "been taken to school by Charlie on several occasions" working opposite Geffen on deals, is equally impressed with Gibson Dunn's strategy.

"It's a great firm, but its reputation in the UK has primarily been built on litigation," he says. "They've really managed to slowly turn that aircraft around in the last couple of years, and are starting to make some smart investments."

One unknown quantity is whether Geffen's role in management will have dulled his edge, given the need for private equity dealmakers to work at the coalface full time.

However, Ashurst sources say Geffen kept a number of relationships very close throughout his time in management, leaving the door open to his previous clients – Apax Partners, Cinven, Warburg Pincus and Blackstone among them.

According to another former colleague, during Geffen's time as the head of the private equity practice between 1999 and 2009, he brought in more revenue to the firm than any other partner in its history – and by some distance.

For Ashurst, the departures will again raise questions about its corporate practice, which ten years ago was perceived as top of the pile. And while Geffen's exit had been expected, Sperotto's was not, and will be felt as a further sign that the practice's self-belief is slipping away.

Sperotto, who became an Ashurst partner in 2007 and helped to found the Milan office, is well regarded as a corporate and private equity all-rounder in both the City and the Italian markets. A top biller himself, he was touted as a replacement for former corporate head Stephen Lloyd, after the latter stepped down from the post in the wake of last year's chairman election, later joining Allen & Overy (A&O).

Together, the hires are being viewed by Gibson Dunn as the largest coup for the London office since the 2008 appointment of Lord Falconer, the former UK Chancellor and Justice Secretary.

For Ashurst, the London office will have seen little of Geffen in the last year, and so many may feel that his lengthy chapter in the firm's history had already closed. Still, the departure of a firm leader, who had otherwise expected to remain at his firm forever, is never an easy break-up.

Add to that, there will always be a handful of 'what ifs'. Given he managed to execute the full financial tie-up with Australian outfit Blake Dawson (for all its doubters), might Geffen have been the leader to deliver that illusive US tie-up? News that he and a small group of partners held early stage merger talks with Sidley Austin last year suggests his tenure could have been even more significant, were he given more time.

"Charlie has made a tremendous contribution to the firm over 30 years including five years as senior partner of Ashurst," a firm spokesperson said, following Geffen's resignation today (1 October). "He leaves a considerable legacy and we wish him well."