Investigations have been launched into the spending returns of both lead campaigns in last year's EU referendum, Stronger In and Vote Leave, the Electoral Commission watchdog has announced.

New figures reveal that a total of more than £32 million was spent on the campaign, with the Leave side funded by donations totalling £16.4 million outgunning the Remain side's £15.1 million.

The spending returns show that the Brexit battle was the most expensive referendum ever fought in British political history, said the Commission.

After its initial inspection of spending returns from both sides, the Commission found that neither Stronger In nor Vote Leave had submitted all the necessary invoices and receipts to back up their accounts. The watchdog also said details of suppliers were missing for some payments.

An investigation has also been opened into missing details of suppliers, invoices and receipts in the Liberal Democrats' return, as well as into individual campaigner Peter Harris, who delivered his spending report late and without the required audit form.

The Commission is also undertaking further examination of apparent discrepancies in returns submitted by the European Movement, Ukip, Labour Leave, Grassroots Out and Conservatives In, before deciding whether to open investigations.

The Commission's director of political finance, Bob Posner, 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."

A Vote Leave spokesman said: "Vote Leave's accounts were approved by external auditors and we believe we have fully complied with all the spending regulations for the EU referendum.

'Vote Leave will fully co-operate with the Electoral Commission's investigation. When we handed in our return on 23 December, we realised that we had submitted it with excess spending that did not need to be reported so it was therefore amended. We were also missing a handful of invoices from suppliers but these have since been provided.'