Media playback is unsupported on your device Media caption Mr Clegg says he is determined to "make people at the top make a fair contribution... to balancing the books"

Nick Clegg has insisted the coalition will not make future spending cuts "on the backs of the poor" and urged the Conservatives to back a "wealth tax".

The Lib Dem leader said those with most assets should "pay their fair share".

He also told BBC One's Andrew Marr Show he would "not flinch" amid growing speculation about his future.

Mr Clegg also unveiled plans to allow people receiving large sums of money on retirement to use them to help underwrite their children's mortgages.

Work and pensions minister Steve Webb has been asked to look at developing such a policy.

The Liberal Democrats are holding their annual conference in Brighton, with the leadership keen to promote a sense of discipline amid poor opinion poll ratings.

'Want to pay'

The theme of the event is "fairer tax in tough times". Mr Clegg is promising to ensure that the next government spending review will not include further spending cuts without a measure of wealth tax.

He said: "I think many people of considerable wealth in this country want to pay."

He added: "The vast majority of people in this country won't find it acceptable if further fiscal austerity was implemented on the backs of the poor...

"I'm not saying something as big as welfare is immune from further savings but I'm saying that the burden has to be spread fairly."

The Lib Dems already have a policy of imposing a 1% charge - a "mansion tax" - on expensive properties above a threshold of £2m and are investigating further proposals.

But the Conservatives oppose introducing a wealth tax, saying it would be unfair to impose it after a home or other large asset had been purchased.

'Do more'

Mr Clegg said that "so far I have failed" to persuade Prime Minister David Cameron and Chancellor George Osborne to accept his party's arguments, adding: "But the mansion tax is not the only way in which you can make people at the top make a fair contribution to this huge national effort of balancing the books.

"We have already illustrated through capital gains tax, through stamp duty, through tax avoidance and many other measures ... the top 10% pay more and we can do more of that.

"There are numerous ways that we have already done it and numerous ways that you can do more of it."

In an interview with the Mail on Sunday, Chief Secretary to the Treasury Danny Alexander promised an extra 100 HM Revenue and Customs staff devoted to fighting tax avoidance by people with assets worth more than £1m. Previously the threshold was £2.5m.

Earlier this week Mr Clegg apologised for breaking his pre-election promise not to support an increase in tuition fees.

But he said he would not be deterred from the coalition's main stated aim of cutting the budget deficit, telling the Andrew Marr Show: "When you are half-way up a mountain you should not bail out.... I am not going to flinch."

For Labour, shadow chief secretary to the Treasury Rachel Reeves said: "Nobody will be fooled by Nick Clegg's empty words on tax...

"Nick Clegg is the Deputy Prime Minister of this government and he must take responsibility for its actions. After so many broken promises, people will judge the Liberal Democrats on what they do, not what they say."

The latest poll by Opinium, published in the Observer newspaper, puts Labour on 42% and the Conservatives on 30%.

It places the Lib Dems on 8%, behind the UK Independence Party, on 10%.

The poll represents the views of 1,681 people who indicated they were likely to vote from 1,984 online interviews.