Nigel Farage has admitted breaching parliamentary rules by failing to register a visit to the US to headline a fundraiser for Donald Trump.
The Reform leader blamed an error by his office for the failure to disclose information on the trip to Florida in March to appear as a special guest at a $500-a-head Republican Party event in Tallahassee.
The Sunday Times reported that Mr Farage did not disclose who paid for the visit, and how much he earned from it.
Under parliamentary law, MPs are required to register visits abroad that cost more than £300 within 28 days if they are not wholly paid for by the MP or public funds. They must also report any fees or payments in kind within the same timeframe.
In a statement issued on Sunday, Mr Farage, MP for Clacton, said: "The trip to Florida in March was remunerated in three separate instalments over the course of two months. I emailed the details to my office in the normal way.
"Unfortunately, these submissions were not added to the register. This was an error. The travel arrangements were paid for by myself.
"A correction to the record will be made tomorrow along with an apology to the registrar."
It was reported at the time that Mr Farage had missed Prime Minister's Questions ahead of his appearance at the event.
Tables for top-tier "Trump sponsors" at the "Disruptors Dinner" were said to have cost 25,000 dollars (£18,440) and include photos with the Clacton MP and tickets to a VIP reception.
Labour and the Liberal Democrats had earlier called on Mr Farage to disclose how the trip was funded.
Anna Turley, chairwoman of the Labour Party, said: "Nigel Farage has failed to disclose who funded this US trip, refused to answer questions about his tax affairs, and changed his story about where he lives.
"Rather than representing his constituents in the UK Parliament to which the people of Clacton elected him, he's been jetting off abroad to call for sanctions against our country, putting British jobs at risk."
Mr Farage has also faced questions over his constituency home in recent weeks.
Last year, following his election as Clacton's MP, Mr Farage claimed in a series of interviews that he had bought a house in his constituency.
But it subsequently emerged the property had actually been bought by his partner, Laure Ferrari, leading to accusations from the Labour Party that he had "misled" the public.
The Reform UK leader told the BBC on Friday he had had a "very expensive week" seeking advice from tax experts and "paid a lot of money to make sure we have done everything right".
Mr Farage denied providing any money for the purchase and insisted the correct amount of tax had been paid.
He said: "The money was legally hers, she bought the house. I don't have any financial stake in it whatsoever - other than she lets me stay there."