Rebel Conservative MPs have put Iain Duncan Smith's leadership under further scrutiny after they defied party orders to vote against government proposals on adoption.

Eight Tories - including former Conservative leadership challengers Michael Portillo and Ken Clarke - joined Labour and Liberal Democrat MPs to deliver a Commons endorsement of allowing unmarried and gay couples to adopt.

Rebel Tories

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

But Health Secretary Liam Fox attacked the rebels' motives in "hijacking" a debate about the welfare of children "to make it about Westminster politics".

He said a group of Tory MPs were seeking to destabilise Iain Duncan Smith's leadership, whether out of "conviction, ambition or cynical self-interest".

The amendment is likely to be overturned in the House of Lords when the legislation returns there on Tuesday.

Monday night's revolt followed the earlier resignation of shadow cabinet member John Bercow, who said he felt he had to step down in principle over the adoption issue.

Questions were also asked about Mr Duncan Smith's decision not to grant Tory MPs a free vote but instruct them to either oppose the government or stay away.

The Conservative leader has recently dismissed talk of "mutterings" at Westminster over a possible leadership challenge.

Rebels speak out

Mr Portillo accused his leader of sending out mixed messages to the public about the modernisation of their party.

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.

In the end six shadow cabinet ministers were among the 35 Tory MPs who did not register a vote.

Mr Portillo: Ruled himself out of leadership race

However, fewer Tories stayed away from the Commons than for the last vote on the issue six months ago.

Mr Portillo argued the Tory leader's stance on adoption was at odds with his conference promise to bring the Conservative Party more in line with the modern world.

But party chairman Theresa May said the debate was about putting troubled children first.

Ken Clarke - defeated by Mr Duncan Smith in the final round of the leadership race - backed the rule change when MPs previously voted on the issue.

However, it is believed to be the first time Mr Portillo has voted against a three-line Tory whip.

Mr Bercow told MPs he had stayed away for the last vote but had to follow his conscience this time.

"What we need in his debate is less prejudice and more fairness," he said.

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

'Stable future'

Mr Portillo asked why this issue had been made a three-line whip as he again ruled himself out from standing for the leadership.

Ms May said 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.

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