The Liberal Democrat leader will try to distance himself from the Conservatives by opposing plans for a tax break for married couples.
Nick Clegg is set to spark a row with Tory backbenchers by opposing plans for tax breaks for married couples.
The deputy prime minister will use a speech on the “open society” on Monday to emphasise the difference between the Liberal Democrats and their coalition partners following splits over the EU.
Tory backbenchers are pressing for an election pledge to introduce transferable tax allowances worth up to £150 a year to be implemented in the life of this parliament.
The move, which was personally championed by Mr Cameron in the run-up to the election, made it into the coalition agreement, though with a clause allowing the Lib Dems to sit out any Commons vote.
Mr Clegg will single out the policy in a speech to the Demos think-tank attacking social conservatism.
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, home-making mother – and try and preserve it in aspic.
“That’s why open society liberals and big society conservatives will take a different view on a tax break for marriage.
“We can all agree that strong relationships between parents are important, but not agree that the state should use the tax system to encourage a particular family form.
“Conservatives, by definition, tend to defend the status quo, embracing change reluctantly and often after the event.”
Mr Clegg will argue that liberal values are more important than ever as the world faces deep economic uncertainty and risks turning inwards.
“The danger in the UK is that the forces of reaction and retreat overwhelm our instinct for openness and optimism. That we succumb to fear – the greatest enemy of openness – in these dark economic times,” he will say.
We need to get away from the idea that there is something on a piece of paper that says if you are married that’s good and if you’re not married it’s not. Nick Clegg
Mr Clegg told Sky News that a tax break was not only philosophically objectionable but also unlikely to influence people’s decision whether to get married.
“Most people get married because they love each other not because they’ve looked at tax returns and seen that they are going to get some cash back from the state.
“We need to get away from the idea that there is something on a piece of paper that says if you are married that’s good and if you’re not married it’s not.
“At the end of the day, deciding to get married should be, in my view, something which is always regarded as a private decision. It isn’t really the business, philosophically from my point of view, to say: ‘Look here, you are going to get £20 back, is that going to help you get married?’. I happen to think it wouldn’t make much difference to people’s decision anyway.”
Labour deputy leader Harriet Harman said: “Despite Nick Clegg’s attempt to spell out differences with the Tories, his actions tell a very different story.
“He has made an active choice to prop up a Tory-led Government which is hitting families harder than the banks through cuts which go too far, too fast, which has cut support for working families and which has increased VAT – costing a family with children £450 a year.
“Whether it is trebling tuition fees, taking a backseat in Europe or giving up on electoral reform, it is clear that the Lib Dems have sacrificed their liberal traditions.”