A senior American Congressman has warned that the British Government and the Royal Family could fall in a major overseas intervention.

Ro Khanna, the California Democrat who helped force the release of the Epstein files, spoke to reporters ahead of Ghislaine Maxwell’s closed-door testimony on Tuesday.

Convicted sex trafficker Maxwell fielded questions from a US House committee on Monday – but gave no answer to every single one.

Before that, Mr Khanna warned that Lord Mandelson “may bring down the whole Government, from what I hear”.

“[Mandelson] engaged in terrible behaviour. The allegations are very serious – that he was working for Gordon Brown and then feeding information about UK possibly buying Euros and having Epstein trade on that,” he continued.

Mr Khanna added: “I think this is the most vulnerable the British monarchy has ever been.”

He said “they ought to ask the King and Queen questions” and suggested that “maybe this will be the end of the monarchy”.

Earlier on Monday, the Palace said King Charles had “made his profound concern clear” over his Andrew Mountbatten-Windsor’s links to Epstein.

He even pledged his support to Thames Valley Police if it approached the Crown in any probe into his brother.

Police are also still investigating Lord Mandelson over allegations of misconduct in public office.

It is understood that his position is that he has not acted in any way criminally, and that he was not motivated by financial gain.

But the chaos forced the Prime Minister to face the most serious threat to his leadership on Monday when Scottish Labour boss Anas Sarwar demanded his resignation over the Mandelson affair.

Mr Sarwar described Sir Keir as a “friend” and a “decent man” – but said there had been “too many mistakes”.

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“The distraction needs to end and the leadership in Downing Street has to change,” he told journalists in Glasgow.

The intervention puts Labour’s 37 Scottish MPs on a collision course with Westminster – forcing them to choose between their regional leader and the Prime Minister.

One, Brian Leishman, preempted his local leader by labelling the PM’s position “untenable” on Sunday night.

Scottish Secretary Douglas Alexander, however, joined Cabinet colleagues in backing Sir Keir.

Ministers, led by Deputy PM David Lammy, quickly rallied behind the Prime Minister as Mr Sarwar delivered his remarks.

Transport Secretary Heidi Alexander said Mr Sarwar was “wrong”, while chief whip Jonathan Reynolds cautioned: “Resorting to infighting now does not serve the country.”

No10 Chief of staff Morgan McSweeney and communications director Tim Allan both resigned over Lord Mandelson’s appointment as US ambassador.

That left the PM to address Downing Street staff on Monday morning, urging them to “go forward from here” and demonstrate that politics can be a “force for good”.

“The thing that makes me most angry is the undermining of the belief that politics can be a force for good and can change lives,” he said.

The Government now faces releasing tens of thousands of emails, messages and documents connected to Lord Mandelson’s vetting.

Sir Keir maintains he was misled by the Labour grandee about his relationship with Epstein.