George Osborne has unveiled a multi-billion pound boost to the NHS amid fears of a deepening financial crisis.

The Chancellor has agreed to 'front load' the bulk of a promised £10 billion real terms rise in annual funding by 2020, from 2014-15 levels.

The commitment is part of the reform plan set out by NHS England chief executive Simon Stevens.

The Chancellor was thought to have been resisting calls from Mr Stevens for the cash injection to be brought on-stream quickly.

However, he has confirmed that the Spending Review is to allocate an extra £3.8 billion to the health service in 2016-17.

The settlement means the Government will have made £6 billion of progress towards the target.

Mr Osborne - who is due to unveil full details of the review alongside his Autumn Statement on Wednesday - said: "We promised the British people that their priority was our priority, and we would fund our National Health Service.

"We will deliver £6 billion a year extra investment straight away, as those in charge of the NHS have requested.

"This means I am providing the health department with a half a trillion pound settlement - the biggest ever commitment to the NHS since its creation.

"This will mean world-class treatment for millions more patients, deliver a truly seven-day health service and allow the NHS to implement its five-year plan to transform the services patients receive."

Prime Minister David Cameron said: "This Government is committed to the values of our National Health Service - and with this historic level of investment we are delivering on that by fully backing the NHS's own plan for the future."

Mr Stevens said the settlement would "help stabilise current pressures on hospitals".

He said: "This settlement is a clear and highly welcome acceptance of our argument for frontloaded NHS investment.

"It will help stabilise current pressures on hospitals, GPs, and mental health services, and kick-start the NHS Five Year Forward View's fundamental redesign of care.

"In the context of constraints on overall public spending, our case for the NHS has been heard and actively supported."

Mr Stevens' Five Year Forward View reform plan also involves making £22 billion of efficiency savings in areas such as procurement, reducing waste, and reducing avoidable hospital attendances.

The Government has pledged that by 2020 everyone in England will be able to access GP services in the evenings and at weekends.

By the same time all key hospital services are due to operate seven days a week.

Health Secretary Jeremy Hunt said: "For doctors and nurses working harder than ever on the frontline this upfront investment means we can implement the NHS's own ambitious plan to transform services for the future.

"We are passionate about building an NHS that offers the safest, highest quality care anywhere in the world - with services smoothly operating seven days a week. This new money will help us finish the job."

Anita Charlesworth, chief economist at the Health Foundation, said: "Make no mistake, the NHS is in the midst of an unprecedented financial crisis...

"To stop this decline the health service needs the pledged budget increases to arrive sooner rather than later and the Government's confirmation of front-loaded funding is therefore welcome.

"Funding for NHS England will increase by £8.4 billion by 2020/21 in real terms and a large chunk of that funding will be front-loaded."