Nigel Farage could face punishment from the elections watchdog after failing to declare £200,000-worth of donations over more than a decade.
The Ukip leader apparently disclosed to Brussels authorities that he had been receiving free use of an office in Britain since 2001 - but did not tell the Electoral Commission.
The issue is said to have emerged only when officials saw the barn mentioned in news stories in the run-up to recent local and European polls.
"When we became aware of the potential unreported donations, we were in correspondence with Mr Farage," a spokeswoman for the commission said.
"This then led to the donations being reported to us.
"We have not yet made a decision about whether any further action will be taken against Mr Farage for reporting the donations late.
"We are continuing to review all of the necessary information supplied to us by Mr Farage and are considering it carefully."
Under the Political Parties, Elections and Referendums Act 2000, regulated recipients - including MEPs like Mr Farage - must report gifts within 30 days of accepting them.
Penalties for not complying can include fines of up to £20,000 and, in the most extreme cases, a 12-month prison term.
A Ukip spokesman said: "Every year since 2001, Mr Farage has declared in his European Parliament Register of Interests the use of a rent-free office from J Longhurst Ltd.
"The premises has been used as his MEP office so the European Parliamentary register was the logical place for it to be declared.
"Mr Farage was surprised to learn that the Electoral Commission thought it should be informed as well, as this did not accord with the professional advice he had received at the time."
The Times previously raised questions about why Mr Farage was receiving £15,000 a year in MEP allowances to run a constituency office when he was not being asked to pay any rent on the converted grain store in Bognor Regis.
Mr Farage has insisted it is up to him how the money is used, and denied any wrongdoing or personal profit.
Details released by the commission show he declared 14 benefit-in-kind donations from Mr Longhurst on May 16, dating back to 2001 and totalling around £205,000.