Tax breaks for married couples risk undermining David Cameron’s efforts to modernise the Tories, senior Conservatives are warning.

The Tory leader delighted right-wingers on Tuesday when he welcomed proposals for a transferable tax allowance worth £20 to married couples. He gave a clear indication that he would campaign to introduce the tax break, saying: “My family and my marriage are the most important thing to me.”

But some Tories on the modernising wing of the party feel deep unease at the plans. “For the life of me I cannot see why one should give a prize to people with no children, simply because they happen to be married,” said John Bercow, Tory MP for Buckingham.

The Times has learned that concerns were raised about the marriage tax break at a Shadow Cabinet meeting on Tuesday, chaired in Mr Cameron’s absence by William Hague.

“I think there is a feeling that this is just going to detract from the message on social justice. What about the woman who has left a violent husband? You wonder if this has been thought through,” said a senior Conservative. Another modernising Tory MP said: “This is straight back to the ‘nasty party’. God knows why David has done it.”

By contrast Tory MPs on the Right have heaped praise on Mr Cameron after he embraced the conclusions of the Conservatives’ Social Policy Group, led by Iain Duncan Smith.

“The proposals send a message to the general public that the Conservatives believe that marriage is the foundation of society,” said Julian Brazier, the Tory MP for Canterbury. “Making this the centre of our new agenda is a brilliant move by David Cameron.”

Gordon Brown attacked the plan for a transferable tax allowance yesterday. He accused the Tories of “moralising”. “What do you say to a person who has been widowed? Is it fair for her to lose the married couples’ tax allowance?”

Mr Cameron made clear last night the new proposals would also apply to people in civil partnerships.

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