The spending returns of the Stronger In and Vote Leave campaigns in last year's EU referendum are under investigation, the Electoral Commission has announced.
A total of more than £32m was spent on the campaign - with the Leave side funded by donations totalling £16.4m, outgunning the Remain side's £15.1m.
The spending returns show that the Brexit battle was the most expensive referendum ever fought in British political history, said the watchdog.
Lib Dem spending is also under review.
The watchdog said invoices and receipts submitted by the two umbrella Leave and Remain campaigns appeared to be incomplete and they were seeking further details in relation to certain payments.
While this was disappointing, it stressed that it was too early to say whether any offences had been committed.
"Missing spending details undermine transparency and makes the returns harder for the public to understand," said Bob Posner, its director of political finance.
"Where it appears campaigners have not fulfilled their legal obligations, we have begun and will continue to take action to deal with this."
The watchdog has published details of all campaign spending in excess of £250,000 during last year's referendum battle.
The figures show the rival Leave and Remain campaigns secured the majority of their funding through large donations of more than £7,500 each.
The Electoral Commission has also launched a formal investigation into apparently missing documentation from the Lib Dems and late returns submitted by businessman Peter Harris, who campaigned against EU membership.
It has also said it is examining possible discrepancies in paperwork submitted by the European Movement, Labour Leave, UKIP and Conservatives In.