Nigel Farage, Paul Nuttall and six other Ukip MEPs could face a £500,000 bill after the European parliament launched an investigation into allegations that they misused EU funds.
Officials are looking into whether the eight MEPs broke rules, which ban full-time European parliamentary assistants, who are paid by the EU, from working for a national party.
According to the Guardian, it is understood that Farage and fellow Ukip MEP Raymond Finch will be asked to repay around £84,000 paid to their joint assistant, Christopher Adams.
Adams is also Ukip's national nominating officer and financial controllers at the EU are investigating whether there was a conflict of interest between his EU-paid job and national party role.
It is also understood that three assistants, who worked for Nuttall, are among those under investigation.
Nuttall is in contention to become Ukip's second MP in Westminster in the Stoke-on-Trent Central by-election later this month.
Ukip has a total of 20 MEPs in the European parliament. Each of the European parliament's 751 MEPs is entitled to €23,400 (£19,903) a month to pay for staff in Brussels and their home constituencies, plus a further €4,342 for office expenses, the Guardian reported.
If Ukip MEPs had claimed the full allowance in 2016, the total sums involved would have been €7.3m.
Farage's wife, Kirsten Farage, is also said to be caught up in the inquiry because she was paid as an MEP assistant while running her husband's office for the national party.
Ukip vehemently denied the allegations and said it would be appealing any judgements made.
A spokesman said: "We have been here [as elected MEPs] since 1999 and have scrupulously applied the rules of the European parliament with very few problems.
"It would appear that post-Brexit there is an element of vindictiveness in the way in which the European parliament is behaving.
"We are appealing each and every one of the allegations that have been made."