George Osborne said selling off the 30 per cent stake would raise £1.5bn more to reduce the debt by a total of £4.5bn.

The £3bn in "extra savings" at various Whitehall departments this financial year are on top of the £13bn they were already making.

Treasury aides said the cuts were a "first step" and equivalent to around 3 per cent of spending outside the protected areas of the NHS, schools and international aid.

The cuts include:

• £450m from the Department for Education - it was specified this will not come directly from schools
• £200m from the Department of Health - it was specified this will not come directly from the NHS
• £500m from the Ministry Of Defence
• £545m from the Department of Transport, including the sell-off of shares in land around King's Cross worth £345m
• £450m from the Department for Business, Innovation and Skills (BIS)
• £230m from the Department for Communities and Local Government

The Chancellor said it showed the Government was "getting on with what we promised."

"When it comes to living within your means, the sooner you start, the smoother the ride," he said.

Mr Osborne said the Royal Mail stake would be sold off within months. The announcement comes two years after the initial flotation.

"It is the right thing to do for the Royal Mail, the businesses and families who depend on it - and crucially for the taxpayer," he said.

But Shadow Chancellor Chris Leslie MP said: "It sounds to me that any semblance of a long-term plan has been totally ripped up, that we've got panic in the Treasury, that we've got chaos with in-year public spending decisions taken. 

"Why didn't he announce those in the March budget if they were part of some sort of a long-term continuum?

"Has he suddenly decided to rapidly change his course when it comes to public expenditure? And why do it in such a shabby way?"

Sky News Political Editor Faisal Islam said: "Much of these cuts come from looking at where departments had under-spent before, but essentially this is about a few of the bigger departments coming up with a few hundred million each in order to help quicken the pace of deficit reduction and deliver overall £30bn in cuts by 2018.

"What I understand is that there will be specific cuts, it's not just about 'efficiency savings'.

"There will be some funding cuts to the Higher Education Funding Council, to the Skills Funding Agency, to the health funding for local councils for the campaigns they do on public health.

"It's just a taste of things to come."

A Department of Health spokesman said: "The NHS budget will remain protected but difficult decisions need to be made right across Government to reduce the deficit.

"Local authorities have already set an excellent example of how more can be done for less to provide the best value for the taxpayer.

"A consultation will now be held with them to decide the best way of delivering the savings that need to be made."

A BIS spokesman said: "BIS will be making a contribution of £450m to the overall savings target - around 5% of its non-science resource budget.

"A significant proportion of the savings will be found through surrendering underspends, making efficiencies and reducing lower-value spend.

"Priority areas for growth and productivity, including the science ring fence and apprenticeships, will be protected."

Commenting on the Chancellor's announcement of the sell-off of land around the King's Cross, Labour London Mayoral hopeful Sadiq Khan said: "London is facing a chronic housing crisis, with land being bought and sold at a premium.

"The Chancellor and the Government should not be making a fast buck out of land they own but handing this over to the people of London so we can build affordable housing to buy and rent for future generations - anything else will be a betrayal of Londoners."