Sky News understands that Heidrick & Struggles has been enlisted by the state-backed bank's board, weeks before it publishes its annual results.

City sources said that Heidrick's appointment had been made with the support of Rick Haythornthwaite, NatWest's chairman-designate, who joined the board earlier this month and takes over from Sir Howard Davies in April.

The search for a permanent successor to Dame Alison, who left last summer amid the furore created by her inaccurate briefing to a BBC journalist about former UKIP leader Nigel Farage's finances, is expected to reach a swift conclusion, according to shareholders in the bank.

Chancellor Jeremy Hunt has outlined plans to offer a significant chunk of the government's remaining 36% stake in NatWest to ordinary investors through a retail offer, with the general election timing and the bank's financial calendar meaning that a mid-year sale is likely to be the only viable window to do so.

Having a new chief executive in place is viewed as being essential for such a sale to happen.

Paul Thwaite, the former head of NatWest's commercial banking business, has stepped in to replace Dame Alison on an interim basis, and fellow executives regard him as a credible choice to land the job permanently.

The government has been steadily reducing its holding in recent years, having at one stage owned more than 80% of what was then called Royal Bank of Scotland Group.

British taxpayers injected £45.5bn into RBS in 2008 to prevent a collapse which would have had dramatic consequences for the wider global banking system.

NatWest declined to comment.