THE Liberal Democrats have urged the Conservatives to "come clean" about their plans to cut benefits by £12 billion.

Lord Scriven, the party's General Election spokesman, claimed David Cameron either did not know where the axe will fall or "he is not being honest" with the public.

The Prime Minister came under pressure to reveal more details about the Tory plans for the welfare budget for working-age people after conducting a round of broadcast interviews on day two of the election campaign.

Mr Cameron insisted the party has been clear about the savings it has identified, although adopted a similar approach to his senior Tory colleagues when pushed for further details.

Work and Pensions Secretary Iain Duncan Smith previously suggested it was not "relevant" to explain where the cuts would fall while Chancellor George Osborne confirmed details would not be published under after the General Election.

The right-leaning think tank the Taxpayers' Alliance (TPA) insisted a "grown-up debate" about Britain's public finances was needed ahead of the poll.

It also reiterated how the welfare savings could be made by 2017/18, including a £20,000 welfare cap and cuts to child tax credits.

In a statement after Mr Cameron's interviews, Lord Scriven said: "David Cameron either doesn't know where the axe will fall, or he is not being honest with the public.

"Either way he is riding roughshod over some of the poorest people in society.

"It is time for the Tories to come clean about their plans to cut £12 billion in working-age benefits from the welfare budget, which will hit 8 million low income families.

"The Conservatives might not think it's 'relevant' to say whether they would tax disability benefits or restrict child benefit to the first two children but the people who rely on this support do."

Mr Cameron was repeatedly pressed over Tory plans to cut £12 billion from benefits for working-age people, which is part of an overall £30 billion consolidation package.

It also includes £13 billion from public spending cuts and £5 billion from a crackdown on tax avoidance.

Asked whether he could rule out achieving some of the savings from cuts in disability benefits, he told the BBC that the coalition had just introduced a new Personal Independence Payment, adding: "We haven't created that benefit in order to undermine it. We want to enhance it and safeguard it."

He added: "Politics is about choices and my choice - and I think the British people's choice - is to find savings in welfare, rather than to put up people's taxes."

Mr Cameron also told ITV1's Good Morning Britain: "Effectively what we are saying is that we need to save £1 out of every £100 that the Government spends in the next two years. We've done that in the last five years. We need to do it for two more.

"I think the right choice is to try to find that saving of £1 in every £100, rather than putting up people's taxes or adding to our borrowing.

"Where we've identified savings, we've been very clear about them. For instance, we've said that we'll cut the welfare cap - the amount of money that any family can get in welfare - from £26,000 to £23,000."

For the TPA, chief executive Jonathan Isaby said: "Politicians must come clean with voters about their spending plans in the next Parliament.

"If they're as keen on balancing the books as they say, they will need to find savings - and yet the election campaign has instead been marked with naked bribes and mudslinging.

"Taxpayers deserve a grown-up debate about the future of Britain's public finances, and that means those seeking our vote on May 7th must be more upfront about their plans.

"Westminster rarely gives the public enough credit and perhaps if politicians showed real backbone and were honest with voters, they might be rewarded."

The TPA referred to its The Spending Plan research, which it said indicated more than £12 billion could be saved from the welfare budget by 2017/18.