Three former Conservative health secretaries have criticised Iain Duncan Smith's hardline stance over the adoption of children by unmarried couples.
Stephen Dorrell, Virginia Bottomley and Kenneth Clarke are all likely to defy an instruction from party whips to vote against the proposed reform when it comes back to the Commons this week.
Mr Duncan Smith and the shadow Health Secretary, Liam Fox, have imposed a three-line whip in both Houses of Parliament in an attempt to block a measure which many Conservatives see as a threat to the institution of marriage.
Under current law, unmarried people can adopt children as individuals but not as couples. Last May, MPs voted by two to one to allow heterosexual and gay couples to adopt, if they pass the screening procedures.
Mr Duncan Smith's opposition has risked splitting his own party, with a substantial number of Tories indicating that they will either abstain or vote against their party.Mr Dorrell said he would "probably" stay away when the vote is held on Tuesday. He said: "I am essentially in favour of allowing unmarried couples to adopt, provided all the proper checks are made. The argument that individuals adopt, and that therefore the change isn't necessary, is not one that I find persuasive. Individuals should be able to make these decisions for themselves."
Mrs Bottomley, who supports adoption by unmarried heterosexual couples, said that she was "reflecting" on how she would vote. She added: "I think the interests of the children should come first."
Felicity Collier, chief executive of the British Association for Adoption and Fostering, met senior Conservatives, including Mr Fox, last week in a private attempt to persuade them to change their line. "It was a friendly meeting," she said. "I said 'Please, please will you change your position?' He said: 'I cannot and we will not.'"
Iain Duncan Smith's policy will come under fire again this morning, from Stephen Norris, expected to be Tory candidate for Mayor of London. In an interview for GMTV, he warns that the vote would undermine efforts to make the Conservative Party appear more in touch with the times.
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