The Parliamentary Standards Commissioner is launching an inquiry into whether Reform UK leader Nigel Farage broke Commons rules by accepting a £5m gift and not declaring it, the BBC understands.
Farage has said he was under "no obligation" to declare the gift from billionaire Reform backer Christopher Harborne because it had been given before he was an MP.
But Reform's opponents say he should have declared it in the MPs' register of interests when he was elected to Parliament in 2024.
The Conservatives wrote to Parliament's standards watchdog, which is now investigating whether the Reform leader broke the House of Commons code of conduct.
A spokesman for Reform UK said: "Mr Farage's office is in communications with the Parliamentary Commissioner for Standards.
"He has always been clear that this was a personal, unconditional gift and no rules were broken. We look forward to this being put to bed once and for all."
Labour Party chair Anna Turley said: "Nigel Farage has been avoiding legitimate questions since news of his billionaire backer's 'gift'.
"It's right that he faces a proper investigation."
A Conservative Party spokesman said £5m was "more than most people will earn in a lifetime".
"Nigel Farage needs to explain how he got it, why he got it, and why he didn't declare it," they added.
"If there is a simple answer then he should welcome these investigations. But like so often with Reform, there is something very fishy about the whole story."
Farage has said he was under "no obligation" to declare the gift from billionaire Reform backer Christopher Harborne because it had been given before he was an MP.
But Reform's opponents say he should have declared it in the MPs' register of interests when he was elected to Parliament in 2024.
The Conservatives wrote to Parliament's standards watchdog, which is now investigating whether the Reform leader broke the House of Commons code of conduct.
A spokesman for Reform UK said: "Mr Farage's office is in communications with the Parliamentary Commissioner for Standards.
"He has always been clear that this was a personal, unconditional gift and no rules were broken. We look forward to this being put to bed once and for all."
Labour Party chair Anna Turley said: "Nigel Farage has been avoiding legitimate questions since news of his billionaire backer's 'gift'.
"It's right that he faces a proper investigation."
A Conservative Party spokesman said £5m was "more than most people will earn in a lifetime".
"Nigel Farage needs to explain how he got it, why he got it, and why he didn't declare it," they added.
"If there is a simple answer then he should welcome these investigations. But like so often with Reform, there is something very fishy about the whole story."