Nick Clegg is to attack David Cameron and fellow Tories who back a tax break for married couples, ridiculing them for wanting to "preserve in aspic" a 1950s view of "aproned, homemaking mothers".
In a speech to the Demos think tank tomorrow, the Deputy Prime Minister will distance the Liberal Democrats from their coalition partners, arguing that they take a fundamentally different view of how society is developing.
After the very public row between the two men over the handling of negotiations to solve the eurozone crisis, Mr Clegg's speech will be seen as a final provocation before MPs begin their Christmas recess on Tuesday. The Lib Dem leader will suggest that Tories have not moved with the times, as attitudes towards marriage, divorce and the role of men and women have changed.
"The institutions of our society are constantly evolving," he will say. "We should not take a particular version of the family institution, such as the 1950s model of suit-wearing, bread-winning dad and aproned, homemaking mother – and try and preserve it in aspic."
Making a distinction between Lib Dems who want an "open society" and David Cameron's "big society" Conservatives, Mr Clegg says the two ideologies take different views on a tax break for marriage. "We can all agree that strong relationships between parents are important, but not agree the state should use the tax system to encourage a particular family form."
A survey for the Centre for the Modern Family found that 77 per cent of people believe single parents can form a family, and 59 per cent thought same-sex couples can be a family.
Before the 2010 general election, Mr Cameron promised a tax break to send "a message" that the Conservatives supported marriage. "I don't think people are going to rush out and get married because there's a certain amount of money on offer every week," he said in January 2010. "I just think that we, as a country, should recognise the importance of committed relationships" to reverse Labour's so-called "couples' penalty". And as recently as last June, he told Tory MPs: "I am a strong supporter of the institution of marriage. I do think it would be a good idea to recognise it in the tax system."
Just days after Mr Cameron gave a speech appealing for a return to traditional church values as "an agenda which speaks to the whole country", Mr Clegg will suggest that it is only the Tories for whom "the emphasis is on non-state institutions such as marriage, the family, churches and voluntary organisations".
He adds: "Liberals pay people the compliment that they know what is good for them, without ideological instruction."
The Lib Dem leader will also seek to revive his social mobility drive by attacking the legal profession for being "woefully unrepresentative" of modern Britain. More than two-thirds of all High Court judges and top barristers are privately educated, he will say. Nine out 10 QCs are men, while 19 out of 20 are white. "I know that we politicians have to get our house in order, too. Not least my own party, which is too male and too pale. We are working hard to fix that. But my message to the legal profession, and especially to the Bar, is: you are not doing enough either. It cannot be right that justice for the many is overseen by the representatives of the few."