An investigation is underway into the spending returns of the two main campaign groups in the EU referendum.

The Electoral Commission said both Britain Stronger in Europe and Vote Leave had not delivered all necessary invoices and receipts, as well as leaving necessary details out of submissions.

Bob Posner, its director of political finance and regulation, 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.

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“Where it appears campaigners have not fulfilled their legal obligations, we have begun and will continue to take action to deal with this.”

He said officials had to call campaigners “to explain their returns” and improve the information given, and would continue examinations until they are satisfied records are complete and accurate.

The Liberal Democrats’ spending return was also missing details including invoices, receipts and supplier names and one group, the European Movement of the UK Limited, declared a total spend of £329,000 but has supplied payment details of only £290,000.

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

Peter Harris, the owner of Butlins, delivered his spending return late and without the required audit report after launching a £500,000 pro-Brexit campaign, and Conservatives IN also gave a donation report after the deadline.

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Grassroots Out Limited also appeared to have missing donation and/or loan reports in its return for spending under £250,000.

After initial inspections, formal investigations have been opened into Britain Stronger in Europe, Vote Leave, the Liberal Democrats and Mr Harris.

Officials are continuing evaluations of six other groups before deciding whether to open full inquiries and the Electoral Commission said it has not yet determined whether any offences have been committed.

It has published the data required by political campaigning laws on returns from 17 registered campaigners, each of which reported spend of more than £250,000.

Mr Posner said the Brexit vote was the most highly-funded referendum ever in the UK, with more than £27m spent.

“Voters must be given an opportunity to see what campaigners at last year’s historic referendum spent their money on in order to secure votes.”