The chief constable of Britain’s second largest police force has warned Theresa May that the service will “break” unless funding cuts end.

In the latest challenge to more years of austerity, the head of West Midlands Police urged the Prime Minister to recognise the “unprecedented security threat”.

Chief Constable Dave Thompson argued it was no longer sustainable for the Government to cut the budgets of individual forces in real terms, as planned.

The warning comes as the Chancellor faces an extraordinary challenge from a string of Cabinet ministers urging him to end the harsh one per cent public sector cap – at a cost of billions.

Philip Hammond must also find huge sums to prevent cuts to school budgets and to ease the ongoing crisis in health and social care.

“Whatever the reasons, West Midlands Police has lost a quarter of its budget. We’ve got an unprecedented security threat,” Mr Thompson told the Birmingham Mail.

“We think the Government needs to offer real terms protection for policing because if you don’t, I think policing is quite unstable nationally.

“Our challenge to Government is to say, we are quite fragile at the moment. You need to stabilise us with real terms protection because otherwise I think policing might break.”

The latest headache for the Prime Minister follows anger over ministers’ claims that police funding has been “protected”.

Earlier this month Damian Green, the de-fact deputy Prime Minister insisted: “There are no police cuts – we have protected police budgets in this parliament.”

However, part of £900m of extra funding over five years is going on priority areas such as cybercrime and tackling child sexual exploitation, which are dealt with at a national level.

That means each police force’s budget will only maintained at current cash levels, without taking into account inflation – which has risen to almost three per cent.

The chief constable pointed out that West Midlands Police is dealing with more attempted terrorist attacks than any force other than the Metropolitan Police, in London.

“The UK has been at a substantial level of threat for a long period of time,” he added. “The threat from ISIS or al-Qaida inspired groups has been a sustained and determined one.

“And what has happened particularly over the last few months is the range of attack planning that has been disrupted - and successful - has been the highest we have seen in the UK since the 9-11 world began.”

The comments echo the warning from Metropolitan Police Assistant Commissioner Mark Rowley - Britain’s leading anti-terror police chief – that funds diverted to fight terrorism could leave the public at risk.

The part of forces' budgets that are paid for by central government has shrunk by 22 per cent on average, since austerity began.