Gordon Brown did not leave the delicate negotiations that brought Quentin Davies into the Labour fold to colleagues or aides. He did it himself.

Mr Davies, MP for Grantham and Stamford, served on the Treasury Select Committee in the 1990s. Mr Brown attended regularly after the 1997 victory and knew Mr Davies from his time as Shadow Chancellor.

Mr Davies got on well with several of his Labour opponents and had good friends, such as Denis MacShane, on the parliamentary ski team. His disaffection with his party’s Euroscepticism has been an open secret for years. But it appears to have been a more general unhappiness with David Cameron’s leadership that sent him over the edge.

The defections of the Blair years were cloak-and-dagger affairs, with his aides meeting MPs at “safe houses” near Westminster to plan their move across. This was different in that the Chancellor was hands-on from the start.

About ten weeks ago, Mr Davies went to see Mr Brown for a chat about various economic issues. During that meeting, and at four further meetings, his disillusionment with the Conservatives became clear and Mr Brown saw his chance.

Mr Davies told The Times that when the meetings began it was increasingly clear to him that he could not remain in the Tory party but far from certain that he would join Labour.

He said: “I was angry that my party was being run in such a cynical and frivolous fashion, but Mr Brown enthused me with the way he was thinking about the big issues of our times and gradually I felt that I could be at home with Labour.”

Two weeks ago, Mr Davies intimated to Mr Brown that he was ready to cross the floor. On Monday the decision was made firm. Dawn Primarolo, the Treasury Minister, was deputed to help him to make the arrangements, although “every single word” of his devastating resignation letter was said to be his own.

Both Mr Davies and the Brown camp insisted that he had made no demands for a ministerial job, although there will be absolutely no surprise if he is sent to the House of Lords as a Labour peer.

For years Mr Davies had been telling Labour friends that he saw no future in a Eurosceptic Tory party. They said that he tried to get Mr Cameron to change tack. The Tory leader’s decision to call for a referendum on the European treaty may have been the last straw.

Mr Cameron was about to address party activists after the grammar school row when an aide warned him that Mr Davies was quitting. He did not mention the defection in his speech and it was only afterwards that he read the letter.

His aides played down the reaction. “We’re not in a blind panic here,” one said. Another insisted that the defection had been “no surprise”, a line later echoed by the leader.

In truth, it comes at a moment of great danger for Mr Cameron. For weeks he has been worrying about how to reconnect with a grassroots uneasy about his leadership. Mr Davies’s critique is likely to resonate with Tories from all sections of the party.

Quentin Davies cuts an unlikely figure as a Labour MP. He is a Lloyd’s name, a member of Brooks’s club and a keen horse rider. He earns a substantial income from directorships and as a parliamentary consultant. He was once the butt of Labour backbenchers’ jokes after being fined for cruelty to sheep on his Lincolnshire estate.

After boarding school, he took a first in history at Cambridge and a fellowship to Harvard before embarking on a career as a diplomat. Seven years later, he joined the merchant bank Grenfell & Co and later becoming president of Morgan Grenfell in France.

He became an MP in 1997 and got his first big break on the front bench under Iain Duncan Smith, who made him Shadow Northern Ireland Secretary, and broke three decades of bipartisan co-operation over the Province by condemning government concessions to Sinn Fein. He has also been hostile to gay rights legislation.

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