Iain Duncan Smith is hoping to re-assert his authority over the Tory Party after a string his MPs rebelled against his tough line on adoption.

Eight Conservative MPs, including former leadership contenders Michael Portillo and Kenneth Clarke, defied a three-line whip imposed by their leader to vote with the Government and allow unmarried and gay couples to adopt.

The shock followed the resignation of shadow work and pensions minister John Bercow, who said he was not prepared to sacrifice his principles to obey the whip.

In the face of the rebellion by Tory modernisers, Mr Duncan Smith is pushing one of the party's traditional policies resurrected from the Thatcher era: giving council tenants the right to buy their homes.

The Conservative leader and shadow secretary of state for the office of the deputy prime minister David Davis are launching a consultation document on the policy at a housing association block in London.

During the adoption debate Mr Bercow launched an attack on those who "lecture people and say you get married and then you're all right", which was "simply not acceptable in the century in which I think I now live".

The Government won the vote 344 to 145. In addition to the eight Tory rebels, 35 Conservative MPs did not vote, although it is not clear how many abstained to allow the proposals to pass and how many were unable to attend the vote.

The rebellion was seen as a triumph for modernisers in the Tory Party and will make difficult reading for Mr Duncan Smith, who has faced speculation that a challenge to his leadership was being plotted among Tory MPs.

Mr Portillo, who last week confirmed there were "mutterings" from Conservative MPs unhappy with Mr Duncan Smith's leadership, has again ruled out mounting a challenge.

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

Tories who voted against the three-line whip and with the government to allow unmarried and gay couples to adopt were: