By Kate Holton

LONDON (Reuters) - Britain's health service is engulfed in a "humanitarian crisis" that requires the support of the Red Cross to use Land Rovers to transport patients, the charity said on Saturday.

Founded in 1948, the National Health Service (NHS) is a source of huge pride for many Britons who are able to access free care from the cradle to the grave.

But tight budgets, an ageing population and increasingly complex medical needs have left many hospitals struggling during the winter season in recent years, prompting headlines about patients being left to wait on trolleys for hours or even days.

The NHS rejected the Red Cross' description and the Department of Health said it had injected an additional 400 million pounds ($490 million) to help with the demand, but the opposition Labour Party called on Prime Minister Theresa May to do more to tackle the overcrowding.

"This is a national scandal," Labour leader Jeremy Corbyn said in a statement.

"We should not have to rely on the Red Cross to provide the basic care the people of this country need. I am demanding that the prime minister comes to the House of Commons on Monday and sets out to the British people how she plans to fix her failure on the NHS."

May's office declined to comment.

The row was triggered by a statement on the British Red Cross website, next to appeals for help in Yemen and Syria, which said it was now "on the front line, responding to the humanitarian crisis in our hospital and ambulance services across the country".

Chief Executive Mike Adamson said the charity was deploying emergency volunteers and having to call on partner Land Rover to lend vehicles to transport patients.

The NHS has always been an emotive issue in Britain - one of the richest countries in the world - and was once described by a former finance minister as the "closest thing the English have to a religion".

In recent years, charities and opposition politicians have warned that government cuts to social care have resulted in more elderly and vulnerable patients being treated in hospital rather than at home, putting a huge burden on the service.

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Keith Willett, a director at NHS England, said the service was not on a par with a humanitarian crisis but said demand was at its higher level ever and staff were under pressure.

"Winter is always a very busy time for the NHS," the Department of Health said. "To support staff working hard on the frontline we have put in place comprehensive plans earlier than ever."

(Reporting by Kate Holton; Editing by Alison Williams)