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Britain's biggest nursing union is threatening the first strike in its 100-year history if David Cameron's 7-day NHS piles even more pressure on staff.

The Prime Minister faces the stark challenge today as he unveils plans for an all-week-round service in his first major speech of the new Parliament.

He'll need 5,000 extra GPs in five years - but doctors claim this doesn't add up because hundreds of GP surgeries have closed despite rising demand.

And nurses are worried they will be forced to work more antisocial hours or lose out on extra payments for working weekends - two things they say are a 'red line'.

The head of the Royal College of Nursing says its 420,000 members could walk out, which they've never done since the body was founded in 1916.

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Dr Peter Carter told The Independent: "Unsocial hours, weekend working, Christmas Day and bank holidays – they get a very modest higher level of remuneration.

"Any attack on that and I do fear it would result in industrial action.”

Health Secretary Jeremy Hunt has already been left floundering on live radio over where 5,000 extra GPs will come from.

He was challenged repeatedly over the source of the staff during a heated interview - and was accused of drifting off into 'vague' answers.

He told Radio 4's Today programme: "We need to look at why it is that, um, we, er, that GPs have so much burnout.

"We need to look at the GP contract, we need to look at what it is that's putting people off becoming GPs."

When told to give more detailed information he snapped: "You're asking me to give you the entire answer now in two minutes."

But GP Matthew Cheetham told the show: "Who is going to be staffing these extra shifts? The government is talking about 5,000 extra GPs by the end of this Parliament. Where are they going to come from?"

In September it was revealed more than 500 GP surgeries had closed since the Coalition took power - and just 110 opened to fill the gap.

And the health secretary was read a letter from a listener in Chesterfield, Derbyshire, who complained two out of every five surgeries had shut down.

Mr Hunt, however, accused the nurses' union of 'jumping the gun' just '8 days into a new government'.

(Image: PA)

He told BBC Breakfast: "I think the RCN should talk to their members and, rather than grandstanding like this, should come and talk to me."

David Cameron will outline the plans later as he renews his pledge to plough an extra £8bn into the NHS.

Speaking at a GP surgery in the West Midlands, the Prime Minister will say: "Our commitment is to free healthcare for everyone - wherever you are and whenever you need it.

"That means getting the best care and making that care available for everyone - free - wherever they are and whenever they need it.

"So I believe that together - by sticking to the plan - we can become the first country in the world to deliver a truly seven-day NHS."