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."
The Commons code of conduct states that new MPs "must register all their current financial interests, and any registrable benefits (other than earnings) received in the 12 months before their election within one month of their election".
The rules say "purely personal gifts or benefits" from family or commercial loans would not normally have to be registered.
The rules also say "both the possible motive of the giver and the use to which the gift is to be put should be considered", adding "if there is any doubt, the benefit should be registered".
MPs who are judged to have broken the code of conduct face a range of punishments from a written or oral apology to suspension from the House or even expulsion, in the most serious cases.
In January, Farage was found to have failed to register £384,000 in interests on time.
He was allowed to update his register via the "rectification" procedure without sanctions, which standards commissioner Daniel Greenberg concluded had been "inadvertant".
The Conservatives have also raised concerns with the Electoral Commission, which said it was considering the information.
