The money could be used to pay someone else to look after a relative or friend, help fund a joint holiday away for both carer and the person they look after, or allow a carer some time on their own at home.

The sum provided would be equivalent to the weekly charge at a care home, but could be used as the carer wished.

The policy, for those who care for others unpaid for at least 50 hours each week, will be unveiled by the Lib Dem leader, Nick Clegg, during a speech to the King's Fund health thinktank in London.

It comes after days of inter-party ­feuding over future funding for care of older people, when Labour attempts to obtain a consensus on principles that might provide a basis for future policies were torpedoed by the Conservatives attacking a so-called "death tax".

Andy Burnham, the health secretary, is thought to favour a progressive inheritance tax rather than a flat rate sum.

Clegg will accuse the Tories of killing one of the best hopes in years of agreeing a long-term settlement on the politically sensitive issue of care for the elderly, but also criticise the government for its ­"cynical promise" of free care at home, which he will say would actually lead to a cut in care budgets.

"It is time to recognise there is a hidden army of people in Britain, without whom no social care policy would be even remotely affordable. Between them they save an estimated £87bn a year," Clegg will say.

Those who cared for loved ones for more than 50 hours a week unpaid were working more than those in paid employment "without even the right of time off, breaks or holiday".

Clegg will say that the party's "simple promise" will enable carers the time they "so desperately need to rest, recuperate or simply have a holiday".