Both Leave and Remain groups under EU referendum campaign spending investigation

Campaign groups from both sides of last year's EU Referendum vote are to have their spending returns investigated by the Electoral Commission. Around £31m ($39m) was spent by various campaign groups ahead of the vote – £16.3 on the Leave side and £15.1 for Remain – making it the most expensive referendum ever fought in British politics.

However, two of the lead campaign groups, Britain Stronger in Europe and Vote Leave, are under investigation by the commission as they do not appear to have delivered all the necessary invoices and receipts to support their returns.

The Liberal Democrats' spending return also appears to be missing some details including invoices, receipts and supplier names.

Elsewhere, European Movement of the UK Limited said it spent £329,000 but has only payment details of £290,000, according to the commission.

Labour Leave and the Ukip submitted returns with discrepancies in the way they reported the same campaigning activity.

Butlins owner and Leave advocate Peter Harris is also under investigation after he delivered his spending return late and without the required audit report.

Bob Posner, the Electoral Commission's Director of Political Finance and Regulation & Legal Counsel, said: "It is disappointing that some campaigners, including both lead campaigners, appear to have not fully reported all their spending as they should have.

"Missing spending details undermines transparency and makes the returns harder for the public to understand. Where it appears campaigners have not fulfilled their legal obligations, we have begun and will continue to take action to deal with this.

"Our statutory role requires us to publish spending returns provided to us by campaigners which we expect to be in a fit state. To make a number of these ready for publication, the commission has had to work with some campaigners to improve the quality of information originally submitted. This has included calling campaigners in to explain their returns. The commission will continue to examine campaigners spending returns until it is satisfied that they are complete and accurate."