Former Conservative leadership challengers Michael Portillo and Ken Clarke are among eight Tory MPs who have defied party orders and voted to allow unmarried couples to adopt.

The move will be seen as another blow to Tory leader Iain Duncan Smith, who had ordered his MPs to oppose the government's proposed change to adoption rules or stay away from the vote.

Rebel Tories

John Bercow Ken Clarke David Curry Julie Kirkbride Andrew Lansley Andrew Mackay Francis Maude Michael Portillo

Earlier on Monday shadow cabinet minister John Bercow resigned in principle over the issue.

The House of Commons voted by 344 votes to 145 to overturn opposition to the law change from the House of Lords, where the legislation now returns.

Andrew Lansley, one of the rebel Tory MPs, told BBC Two's Newsnight programme he believed "several more" shadow cabinet members would have voted with the government if Mr Duncan Smith had allowed a free vote on the issue.

As well as those Conservatives actively revolting against the three-line whip imposed by party chiefs, six shadow cabinet ministers were among 35 Tory MPs who chose to stay away and not register a vote either way.

It is not known, however, how many of those MPs abstained intentionally and how many just could not attend.

In the Commons debate, Mr Portillo said Mr Duncan Smith's stance on adoption was at odds with his recent conference promise to bring the Tories more into line with the modern world.

But Conservative Chairman Theresa May said the debate was not about whether or not the party was tolerant but about putting troubled children first.

Mr Clarke backed the rule change when MPs last voted on the issue in May, but it is thought to be the first time Mr Portillo has voted against a three-line Tory whip.

Fewer Tories stayed away from the Commons than for the vote six months ago.

Stepping down

The vote means those peers who rejected the change earlier this month have to decide whether to try to prevent it being passed in the last three days of this parliamentary session.

Explaining to MPs why he had resigned, Mr Bercow said he had stayed away when MPs last voted on the issue but felt it would have been a "sell-out" not to follow his conscience now.

Michael Portillo: Defying Tory whips

"What we need in his debate is less prejudice and more fairness," Mr Bercow told MPs.

"We should aspire to govern Britain as she is, not Britain as she was."

Mr Bercow suggested his party needed to be more open minded and understand other people's choices.

The issue at stake was not gay rights, nor the rights of any adults, he argued.

"It is about the rights and welfare and the futures of some of the most vulnerable people in our society," said Mr Bercow.

Mixed messages?

The shadow pensions minister has been replaced by Oliver Heald, who served in John Major's government and who followed the party line in Monday night's vote.

Theresa May says the party is putting children first

Mr Portillo asked Tory spokesman Tim Loughton: "Given that sentiment and given the range of very sincere opinions, can my honourable friend return to the question of why this is a three-line whip?"

Amid rumours of plotting against Mr Duncan Smith, Mr Portillo on Monday again ruled himself out from ever standing for the leadership.

Party officials said ahead of the vote that rebels would be treated leniently.

Theresa May said it was right to have a three-line whip to show what stance the party was taking.

She argued the party was motivated by the desire to provide the most stable future possible for children being adopted.

Married couples were likely to stay together longer than unmarried couples, she said, and therefore provided a more stable background, she said.

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"This is not a question of whether or not the party is a tolerant party - the party is indeed a tolerant party," she added.

Liberal Democrat leader Charles Kennedy said the row showed the Conservatives were a "shambles of an opposition party".

He accused Tories of "posturing" and trying to score political points on a serious issue.

But the Tory decision not to force votes as MPs overturned two other changes forced by the Lords could signal they were now "seeing sense", he argued.

It is understood there was a row at the recent Tory shadow cabinet meeting where the party line was decided.

Mr Bercow was the most vociferous supporter of the liberal amendments with David Davis and Michael Howard being the staunchest defenders of the status quo.