How a tactical error by Duncan Smith led to a bad call

The call, when it came, was a shock. Iain Duncan Smith was being driven to Bedford for a live interview on Jonathan Dimbleby's programme on ITV, his head full of arguments on everything from Tory health policy to plotting MPs.

Ironically, one policy line that he wanted to get clear was his party's three-line whip against so-called gay adoption, a subject he knew was dividing his Shadow Cabinet.

An aide took a call from David Maclean, the Opposition Chief Whip, who said John Bercow, the shadow Work and Pensions minister, was threatening to quit. At about 10.45am, Mr Bercow phoned Mr Maclean to tell him that he could not absent himself from the vote due last night.

Given Mr Bercow's repeated insistence on raising the issue of gay adoption, the Conservative leader knew that he was facing problems. But he had no idea that one of his best parliamentary performers was prepared to stand down.

Staff at Bedford High School, the location for the Dimbleby interview, said that Mr Duncan Smith looked nervous and flustered when he arrived, late, for the show. They thought it was because his car had taken a wrong turning. By yesterday, the reason was clear.

During the hour-long interview, the Tory leader went on the offensive when confronted about adoption. He was applauded for stressing the rights of the children to be adopted, rather than the rights of gay couples who wanted to adopt. But afterwards, in a lengthy phone call, he tried to persuade Mr Bercow to stay. Mr Bercow refused, although he stressed that he would present his resignation as a single issue matter of conscience, not a general attack on the leader.

To limit the damage, the Buckingham MP also agreed to keep his resignation secret until yesterday and refused to make any comment apart from a statement in the Commons.

Nevertheless, his resignation letter was made available to London's Evening Standard by midnight on Sunday. It included a barbed reference to Mr Duncan Smith's days as a rebel under John Major. "I have long admired your political courage and integrity. In particular, your battle against the Maastricht Treaty was inspirational," he wrote.

The die for this resignation was cast back in May, when right-wingers in the Shadow Cabinet executed an extraordinary manoeuvre to block gay or unmarried adoption.

Mr Duncan Smith had convened one of his usual morning meetings when the issue was raised. Crucially, Oliver Letwin, the shadow Home Secretary, was absent. David Davis, then chairman of the party, and Michael Howard, the shadow Chancellor, seized their moment. They persuaded the leader that a three-line whip was the only stance to take on the forthcoming Children and Adoption Bill. Mr Duncan Smith agreed. By the time the rest of the Shadow Cabinet heard of the decision, it was too late. The following meeting made clear the deep divisions on the issue, but the leader decided that there should be no free vote. Several shadow cabinet members absented themselves and key figures such as Kenneth Clarke and Mr Portillo went further and voted against their leader in the division lobbies. Mr Bercow stayed in his room in Portcullis House.

When the Shadow Cabinet debated the matter again last month, Mr Bercow objected to the three-line whip. Mr Letwin backed him, but David Willetts and Theresa May did not. Mr Duncan Smith closed the discussion with a firm order: the position would not change.

As the impact of Mr Bercow's decision reverberated last night, this was clearly not just a local difficulty over a moral issue by a single MP. With Michael Portillo leading the biggest Tory rebellion of this Parliament, Mr Duncan Smith's authority was dealt a humiliating blow.

With Mr Duncan Smith under pressure from "mutterings" about a leadership challenge by backbenchers, the gay adoption issue has crystallised his critics' worries about him. "He may be a moderniser in his head, he knows it has to happen, but in his heart he just hasn't got the instinct for it. Scratch the surface and his unease about gay couples and unmarried couples pours out," one senior figure said. "It's as simple as that."