The bombshell report comes just a day after Aslef rail barons announced a wave of fresh walkouts – 48 hours after being offered a bumper increase.

It sets Britons up for another autumn of misery with Heathrow’s Border Force guards also saying they will strike from the end of the month.

Teachers, GPs and other rail unions are also set to push for more money, despite inflation-busting pay rises.

The £10 billion reserved by the Chancellor ­followed recommendations from independent pay review bodies.

But the new analysis by the Conservative Party reveals this year’s settlements are set to ­balloon by another 40 per cent – to £13.9billion.

The figure draws on briefings from the former government’s negotiations with unions, including teachers, who have publicly only said they want more than the promised 5.5 per cent.

It includes the new Aslef offer, the 10.7 per cent being demanded by GP unions, and the cost if Labour accepts the RMT rail union’s demand for a 14.25 per cent deal to match Aslef’s.

The report also assumes ministers will cave in to the Royal College of Nursing’s demand to get ‘the same fair treatment’ as junior doctors – who were offered a 22 per cent rise over two years.

However the figure does not include the cost of settling with passport control staff, who have not yet proposed a number.

The extra £4billion would be almost equivalent to a 1 percentage point National Insurance tax rise, which costs £4.7billion according to official data.

A source who was previously involved in negotiations said: ‘They see a new government more favourable to unions and are naturally asking for more.’

Tory MP Alicia Kearns told the MoS: ‘Let’s be real – Labour’s true loyalty lies with their union paymasters.

‘They promised to end strikes yet, mere days after agreeing bumper pay increases for train drivers, Aslef are back threatening fresh industrial action.

‘Of course Aslef are the biggest donor to the Labour Party – so no surprise they get preferential access to Labour treating us all as cash machines.

‘Now the secret’s out, every union boss will be circling. After all, they made donations and what’s the bill? A whopping £4billion!’

Questioning how Labour will fund it, she asked: ‘[Will they cancel] our new Aukus submarines? I wouldn’t be surprised given their Cabinet’s record on defence.

‘They’ll come for us all with tax rises in time, just as we knew they would.

‘Is this a return to the dire days of the 1970s, when the UK was the ‘sick man’ of Europe? Wage-price spirals leading to rampant inflation, a Labour administration grovelling to its union benefactors, and incapable of saying no?’

Shadow business secretary Kevin Hollinrake added: ‘These are nothing but shabby deals cooked up by Labour’s naive ministers and their union paymasters.’

It comes as it was revealed that Cabinet ministers have received more than £500,000 in union donations in the past five years.

Of these, Deputy Prime Minister Angela Rayner has the highest number – more than £144,000 since 2019.

Last week Transport Secretary Louise Haigh approved a 14.25 per cent pay deal for train drivers over three years without any concessions on reforming the system.

However, on Friday, the Aslef union announced further rail strikes on the East Coast Mainline, connecting London to Edinburgh.

Members will take action for 22 days – every weekend from August 31 to November 10.

Mick Lynch, boss of the rival RMT union – which represents train guards, station staff and signallers – said he wants a ­similar deal for next year despite previously accepting a 4 per cent rise.

Ms Haigh is now under pressure to withdraw the Aslef offer if it goes ahead with the planned strikes – although the union insists the walkouts are separate from the pay dispute.

Labour-affiliated Aslef, which has a seat on the party’s powerful National Executive Committee, has donated £180,000 to Labour since Sir Keir became the party’s leader.

Also on Friday, border guards at Heathrow said they will strike from August 31 to September 3.

Conservative Party chairman Richard Fuller said: ‘This new Labour government is recklessly handing over cash to their union paymasters with ‘no strings’ attached.

‘But in doing so they are sending a dangerous signal to unions across the country that you can strike and be rewarded with a bumper pay packet with no improvements for consumers.’

He also accused the Government of abandoning pensioners after the Chancellor scrapped the winter fuel allowance for those not on benefits, adding: ‘Rachel Reeves is showing she is a ‘soft touch’ when dealing with union bosses but cold-hearted to pensioners.

‘Now with the extra bill heading to billions of pounds, be in no doubt that Labour will soon be putting their hands in your pocket to pay for it.’

Caroline Abrahams, of charity Age UK, said: ‘The media is full of stories about efforts to bring some long-standing industrial disputes to an end and the Government’s apparent willingness to use significant amounts of public funding to achieve this.

‘Some of the pensioners who are getting in touch with us are telling us they feel that their needs are being considered as of lesser importance, as a result.’

Paymaster General Nick Thomas-Symonds has defended the Aslef pay offer, which follows two years of crippling strikes, and said it would ‘cost more not to settle’ many of the disputes the Government is facing.

Labour has refused to say how much taxpayers will have to contribute towards the pay deal for train drivers, which will cost more than £100million.

Shadow Chief Secretary to the Treasury Laura Trott said: ‘Labour need to be honest that they are prioritising unions over pensioners, and have wanted to raise taxes all along.

‘The British public will not forgive Labour if they break their promise not to raise any taxes on working people.’

A Labour spokesman said: ‘The last Government presided over the worst set of strikes in a generation, which caused chaos and misery for the British public and wreaked havoc on the public finances.

‘Industrial action in the NHS alone cost the taxpayer £1.7billion last year.

‘We value the vital contribution the almost six million public sector workers make across the UK, delivering the public service we all rely on.’