Iain Duncan Smith suffered a new and devastating hammerblow today as one of the leading modernisers in his shadow cabinet suddenly resigned.

John Bercow, the work and pensions spokesman, walked out in protest at Mr Duncan Smith's decision to compel Tory MPs to vote against moves to allow unmarried and gay couples to adopt children.

His letter of resignation, sent yesterday and seen by the Evening Standard, says he is "very unhappy" with the decision to instruct Conservative MPs to vote against the measure.

It says that he is not prepared to go along with the fudge proposed by the leadership to allow dissenters to "arrange to be absent" for tonight's Commons vote. And it says that he intends, instead, to speak out in the debate against his party's line.

The departure of Mr Bercow - at 39 one of the brightest and most active shadow cabinet members - is another calamity for Mr Duncan Smith, already assailed by criticism and doubts over his future.

It also put a direct and damaging question mark over the leader's judgment. It was Mr Duncan Smith personally, who at the end of a long and bruising shadow cabinet wrangle two weeks ago, decreed that MPs must vote against the adoption changes on a three-line whip, the strongest instruction to toe the party line.

The modernisers, including Mr Bercow, shadow home secretary Oliver Letwin, shadow trade secretary-Tim Yeo and shadow defence secretary Bernard Jenkin, had begged for a free vote, on the same lines as that granted to Labour MPs as an issue of conscience. They also warned that barring the right of unmarried couples to adopt children would leave the Tories looking, yet again, like the intolerant "nasty party".

Yesterday morning, Mr Bercow telephoned the party chief whip, David Maclean, and informed him that he could not live with the ruling and proposed to stand down.

Mr Maclean, appalled, urged Mr Bercow to consider the damage his move would do to a leadership already in crisis, and asked him to think again.

The news was broken to Iain Duncan Smith as he prepared to go on ITV's Dimbleby show to declare that all was well with his leadership.

Afterwards, in a lengthy telephone call, Mr Duncan Smith himself pleaded with Mr Bercow to stay. But he refused to budge on the adoption issue, saying the decision had been taken and could not be reversed.

By last night, the resignation letter had been sent. But in one gesture to limit the damage, Mr Bercow agreed not to disclose that he had quit and this morning he was still making no comment.

His letter carries a barbed reference to Mr Duncan Smith's own days as a rebel and a thorn in the side of then Prime Minister John Major, over Europe. It says: "I have long admired your political courage and integrity. In particular, your battle against the Maastricht Treaty was inspirational."

Dozens of Conservative MPs are expected to revolt in tonight's vote, defying Mr Duncan Smith's argument that the most important issue is for adopted children to go in to stable families and that the most stable are those with married couples.

Used courtesy of the London Evening Standard