Theresa May has accused the British Red Cross of being "irresponsible and overblown" after the charity warned that NHS England was facing a "humanitarian crisis".

The Conservative premier made the remarks as she faced a grilling from Labour leader Jeremy Corbyn at Prime Minister's Questions (PMQs) – the first of 2017 – on Wednesday (11 January).

"He talks about the pressures on the NHS and we acknowledge that there are pressures on the National Health Service there are always extra pressures on the NHS during the winter, but of course we have at the moment those added pressures of the ageing population and the growing complex needs of the population," May said.

She added: "He also refers to the British Red Cross' term of a humanitarian crisis.

"I have to say to him that I think we have all seen humanitarian crises across the world and to use that description, of a National Health Service which last year saw two and half more million people treated in accident and emergency than six years ago, was irresponsible and overblown."

The comments come after a 20-strong group of MPs from across the political spectrum co-signed a letter highlighting the "unsustainable strain" on the NHS.

The MPs, including former Liberal Democrat health minister Norman Lamb and Labour's Frank Field, also called on the government to set up a national health and social care convention in a bid to find a long-term solution to the health and social care "crisis".

Lamb said the document would engage patients, the public, civic society, staff, trade unions, and providers of health and care services in a "national conversation about how we guarantee a strong and effective NHS and care system".

"The public is sick and tired of the NHS and care system being treated like a political football. People have had enough, and are crying out for an honest discussion and bold solutions to these challenges," he added.

The Royal College of Nurses, which has more than 432,000 members across the UK, have described current conditions in the NHS as the worst they have ever seen.

UPDATE: 12 Jan (15:50 GMT)

A British Red Cross spokesperson said: "The British Red Cross is part of a worldwide humanitarian network with a mission to prevent and alleviate human suffering, wherever it may be found. We are dedicated to ensuring that people in crisis get the help that they need, whether that's here in the UK or overseas.

"We have over 30 years' experience running health and social care services in the UK and every year we reach over 80,000 people with home from hospital and support at home services. In addition each year we lend out more than 100,000 mobility aids and train thousands of people in first aid.

"Whenever we speak publicly we do so on the basis of humanitarian concern, on the basis of evidence, including what we are seeing from our work on the ground. We witness first-hand the human cost of a health and social care system under exceptional pressure.

"In recent days, we have reiterated our call on the government to ensure sufficient funding for both health and social care - including for preventative services. We have also advocated for the need for a long term, person-centred solution, developed in partnership across parties, professions and other sectors".