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, which is due 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 ten 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 one million in 2015 to 1.1 million by the end of the Parliament in 2020.