Around a million carers would receive an annual "bonus" under proposals being set out by the Liberal Democrats.

Deputy Prime Minister Nick Clegg said the party's 2015 general election manifesto would include a pledge to top up Carer's Allowance by £125 each year, rising to £250 by 2020.

It would be paid to anyone who had been receiving the state help for at least 12 months - about 85% of claimants.

But the party gave no details of how the expected £130 million a year cost of the scheme - rising to £280 million - would be paid for.

A spokesman said the Lib Dems would produce a "fully costed manifesto" before the election next May.

Mr Clegg said the extra money was a recognition of the "superhuman" work done by the "hidden army of carers".

"Carers are Britain's unsung heroes - up and down the country behind closed doors, you'll find a hidden army of carers doing amazing things every day," he said.

"It is estimated that six in 10 of us will at some point in our lives find ourselves caring for someone else - an elderly relative, a sick child, a friend who needs our support.

"Obviously this makes a massive difference to the people being cared for - but Carers UK calculated that it saves us a whopping £119 billion every year - more than the entire NHS budget.

"These are people who deserve our support even in these straitened times, which is why the Liberal Democrat manifesto will include a Carer's Bonus to give them a little extra help.

"It's completely up to the carers themselves how they spend it - it's a no-strings-attached bonus from a party that is grateful for the superhuman work they do."

The party suggested the money could be spent on a weekend break, paying for professional care cover, a cleaner to give the house a spring clean, treats for grandchildren or a new laptop.

The number eligible - also including those eligible for Carer's Premium on top of another benefit - is expected to rise from a million in 2015 to 1.1 million by the end of the Parliament in 2020.

A trust that raises money to fund breaks for carers said many would treat the promise "with some scepticism" after a previous injection of Government funding failed to reach the front line.

Paul Green, speaking on behalf of the Saga Respite for Carers Trust, said: "Whilst it is heartening that Nick Clegg has pledged to give carers a break, the announcement will be greeted with some scepticism by some of the nation's carers.

"In 2010 government pledged £400 million would be allocated for carers to take a much needed break, money they argued would allow family carers to maintain their own health and continue to provide care to their loved ones.

"However, the failure to ring-fence the funding meant that very few local authorities used the funds for this purpose and many carers continued to put their own health and well-being at risk.

"Supporting family carers is common-sense, common decency and should have cross-party support. Carers need action not just rhetoric."

Shadow health minister Andrew Gwynne said: "Nick Clegg and the Liberal Democrats will be judged on their record in Government.

"He wants to talk about carers in the next manifesto, but in truth he made carers' lives harder as when he took away care support for their family, while 60,000 carers have been hit by their unfair bedroom tax.

"He's left social care services close to collapse and a quarter of a million people have lost support in recent years.

"You can't trust a word the Liberal Democrats say."