Defence Secretary Gavin Williamson has threatened to sweep Theresa May from power in a bitter Cabinet row over military cuts.

Furious Mr Williamson warned the Prime Minister that if she did not commit an extra £20 billion to the Ministry of Defence then Tory MPs would vote down the next Budget – effectively passing a motion of no confidence in her.

‘I made her – and I can break her,’ Mr Williamson is said to have boasted to service chiefs.

The furore, described by one Williamson ally as a ‘dogfight at the heart of Government’, erupted after Mrs May announced a £20 billion-a-year boost to NHS spending last week.

Chancellor Philip Hammond then declared there was no money left for similar boosts to defence, housing or schools spending.

Secretary of State for Defence, Gavin Williamson, pictured arriving at Downing Street on Monday for a Cabinet meeting ahead of a keynote speech by the Prime Minister, Theresa May, who has announced plans to give the NHS an extra £384 million a week

When Mr Williamson said he needed his own £20 billion – a minimum of £2 billion a year extra for the next decade – to avoid damaging defence cuts, Mrs May questioned whether the UK had to be a ‘tier one’ military power.

Mr Williamson hit back that after Brexit it would be even more important for the UK to ‘sit at the top table’ internationally.

Last night a formidable array of political and military figures were lining up behind Mr Williamson in his power struggle with No 10.

They were led by the chairman of the powerful Commons defence committee, who warned that Mrs May could be ‘at political risk’ if she did not ‘do the right thing’ by increasing defence spending.

And the former head of the British Army, General the Lord Dannatt, told The Mail on Sunday that he feared Mr Williamson could be forced to resign over the issue.

This newspaper understands that in a recent meeting, Mr Williamson reassured senior members of the Armed Forces that he was fighting for more funds.

According to one source, Mr Williamson promised to ‘crush’ the Treasury over the issue.

When one officer then asked whether the Prime Minister would back him, Mr Williamson – who ran Mrs May’s leadership campaign in 2016 – is said to have replied: ‘I made her – and I can break her.’

Up to 20 Tory MPs are threatening to rebel if the Treasury and No 10 cannot find more money for defence. A powerful delegation formed of Graham Brady, the chairman of the backbench 1922 Committee, along with chairmen of the defence and foreign affairs committees, are planning to meet Mrs May in the coming weeks to urge her to give a generous settlement to the MoD.

One Williamson ally said: ‘There is a large rebellion brewing and there is serious talk of blocking the budget in the autumn unless there is a major investment announced in it for defence. It is the first priority of Government.’

The ally added that Tory backbenchers were ‘incandescent’ that while Mrs May had last week pledged to boost the NHS budget, the ‘more traditionally Tory’ issue of defence has been neglected.

Tory MP Julian Lewis, chairman of the Commons defence select committee, warned last night: ‘There need be no political risk to the Prime Minister – if she does the right thing.

Chancellor of the Exchequer Philip Hammond listens to a speech at the Lord Mayor's Dinner for the Bankers and Merchants of the City of London at Mansion House, Thursday June 21, 2018

‘I’ve been in Parliament for 20 years and I’ve never known such unanimity. If the NHS can get an extra £20 billion a year, then defence surely deserves one tenth of that just to stand still.’

The Williamson ally said ominously: ‘The PM has had no stronger ally than Gavin. He never lost a vote as chief whip and he kept her in her job after the General Election by negotiating the agreement with the DUP. But there’s a strong feeling No 10 and No 11 are both failing to recognise the threats we face.

‘We need investment to maintain Britain as a world-leading military power with influence all over the globe.

‘Gavin is right to fight for the defence budget so our troops can counter the threats and keep Britain as a major world player’.

The increase to the NHS budget has boosted the Tory leadership prospects of Health Secretary Jeremy Hunt at the expense of rivals such as Mr Williamson.

Lord Dannatt accused the Prime Minister of giving Mr Williamson no option but to consider his future. The former head of the Army said: ‘It would appear that the PM is backing Mr Hammond, pushing Mr Williamson into a corner.

'The Chancellor may say he has no more money but he always has options: to increase taxes, to increase borrowing or to trim the overseas aid budget of 0.7 per cent of GDP.

From left, Chancellor Philip Hammond, Health Secretary Jeremy Hunt and NHS Chief Executive Simon Stevens listen to a speech by Prime Minister Theresa May at the Royal Free Hospital in north London where she outlined how spending on the health service will grow by £384million a week in real terms by 2024

‘It would be a disaster for defence if Mr Williamson were to resign on this issue. The MoD has been cursed by too many defence secretaries over the past 20 years.

‘And does Mrs May really want to see Mr Williamson lurking on the backbenches?

‘What the country needs is an increased defence budget providing capable Armed Forces that can enable our country to fulfil its role as a permanent member of the UN Security Council and to substantiate claims for the UK to be truly Global Britain after Brexit.

‘This will sit well with the United States and make President Putin pause for thought.’

During his clash with the Prime Minister, Mr Williamson cited a recent parliamentary report which called for a rise in defence spending from two to three per cent of GDP. The report by the defence select committee calling for three per cent to be adopted as a benchmark is popular on the Conservative back benches.

Donald Trump has been calling on Nato allies to increase the amount they spend on defence. Although the UK is one of the few countries in the alliance to meet the agreed two per cent target, the US ambassador to London yesterday urged No 10 to spend more.

Woody Johnson said: ‘As you become an independent country again, you are going to have to re-evaluate how important security is to you. Without security there is no prosperity.

‘You are going to have to decide what capabilities you require and what the cost is.’

While there is no formal definition of what being a ‘tier one’ power means, defence experts argue that it involves having a full spectrum of military capabilities, including an independent nuclear deterrent and Armed Forces which can be deployed anywhere in the world.

Britain’s defence budget is £37 billion a year. The NHS boost will take the Health Service budget to £126 billion in today’s money.

A source close to Mr Williamson said last night: ‘Gavin is a Defence Secretary who for the first time in a decade is putting up a fight to retain Britain’s place at the top table. There is a dogfight going on at the heart of Government and Gavin is right to defend Britain’s tier one status.’

Mr Williamson was not available for comment last night.