John Bercow has resigned from the shadow cabinet in protest at the Conservative Party's rejection of adoption by unmarried couples.

His resignation as shadow work and pensions minister came as MPs prepared to vote on government plans to allow gay and unmarried couples to adopt children.

While Labour MPs have a free vote on the issue, Tories have been ordered to vote against the measures.

Conservative chairman Theresa May played down the resignation.

"I'm sorry that John feels it necessary to step down from the shadow cabinet on this issue but clearly its an issue on which he feels very strongly," she told BBC Radio 4's Today programme.

"Therefore he feels he is not able to go forward continuing to be a member of the shadow cabinet."

Oliver Letwin, the shadow home secretary, said Bercow's decision was "a pity" - but defended the decision to impose a three line whip.

"We wanted to make sure that the view of the party was clear but we also wanted to allow some room for manoeuvre, and it is what I think is called a soft three line whip. It was made quite clear, widely reported, that people who felt they could not vote for our position would just stay away quietly," he said.

"Of course I understand and respect the very strong opinion that John has expressed. I think it is a pity."

But the move was welcomed by Conservative think tank Connect.

"John Bercow has always been a man of principle and integrity and he clearly feels deeply about this issue. In such cases it is right to do what he has done," said chairman Paul Gray.

"The shadow cabinet should look at this issue again in the light of it's current strategy and adopt a more consistent approach. In the mean time, they should trust Conservative MPs to make their own minds up on this issue and allow a free vote."

Reports indicate that up to 40 Conservatives are tonight set to defy the order to vote against the government's plans.

Iain Duncan Smith has defended the decision to impose a three line whip for the vote, compelling his MPs to reject the plans.

"I don't think that it is about being nice. I don't think it is about being modern or old fashioned," he told ITV1's Jonathan Dimblely programme on Sunday.

"It is about children in need, children in care, not lifestyle choices. As far as I am concerned, children come first, lifestyle choices then come second."

However, he refused to comment on the "stupidity of the ins and outs" about what action he might take against MPs who defy his instructions.

Rebels were reported to include shadow cabinet members Damian Green and Tim Yeo, who may decide to miss the vote.

Other senior Tories, including Michael Portillo, Kenneth Clarke, Stephen Dorrell and Virginia Bottomley, were said to be preparing to back the government's plans.

And former shadow cabinet member Andrew Lansley also confirmed his intention to defy Duncan Smith.

"The signal they think they are sending out is that they wish to defend marriage and that marriage is the most important issue.

Now the adoption legislation is not the right place to engage in any kind of social engineering," he told GMTV's Sunday Programme.

"When you just narrow it down and look at it from the stand point of the child, the best interest of the child, there is no question than that it would be the best thing to do to enable couples of whatever status to be able to adopt."

Senior Conservative Steve Norris also attacked Duncan Smith's position.

"Taking this attitude on this piece of legislation actually confirms all the prejudices of those who see the Tories as out of touch and representing an elderly, angry and frankly rather irrelevant strand of opinion in this country," he told the same programme.

"I think it's an error of judgement to impose this three line whip, and I hope that even at this stage, the Conservatives might reconsider it.

"This will hardly play with the agenda which Iain does say that he's signed up to, which is to present the party as modern and in touch. If anything, this is going in the opposite direction, and it's a mistake."

But the official Conservative position was strongly defended by David Davis, the shadow deputy prime minister.

"If ever there was a group which you might term vulnerable these are they - damaged kids, emotionally and physically damaged, many have suffered abuse, many of them end up in crime, on drugs and so on,'' he told BBC1's On the Record.

"What you want to give them is the most stable environment you possible can - a happy home life. On a statistical basis married couples tend to stay together longer after the arrival of a child. That's hard fact."

Davis added: "I do not think the public at large looking at this will say it is illiberal or irresponsible. They will say they are looking very carefully at what the rights of the youngsters are and doing the best possible for them."

Adding to the Conservative Party's divisions, another former shadow cabinet minister, Andrew Mackay, told the same programme that it was "difficult to explain and understand" why the party was opposing the plans.

"I think the state or political parties telling people how to behave is something which is fairly abhorrent which normally happens only in fairly restrictive regimes of which most of us would not approve," he said.