Rival Remain and Leave campaigners in the EU referendum raised £15.6m in the ten weeks to 21 April, according to the Electoral Commission.
The official campaign for Britain to stay in the EU - Britain Stronger in Europe - raised £6.9m - more than twice as much as Vote Leave's £2.8m.
But the sum raised by all registered leave campaigners was £8.2m - higher than the remain campaigners' £7.5m.
The referendum on whether the UK leaves or remains in the EU is on 23 June.
The figures published by the watchdog cover the period between 1 February to 21 April, detailing money raised by campaigners spending more than £10,000 in the referendum and individual donations of more than £7,500.
Britain Stronger in Europe raised £6.88m, boosted by two donations totalling £2.3m from the supermarket magnate and Labour peer Lord Sainsbury,
Other prominent Remain donors included hedge fund manager David Harding (£750,000), businessman and Travelex founder Lloyd Dorfman (£500,000) and the Tower Limited Partnership (£500,000).
Conservatives In, the party's pro-EU campaign group raised £362,534 while the European Movement of the UK, Michelle Ovens Ltd and Scientists for EU raised £57,494, £95,000 and £60,000 respectively.
Vote Leave, the official Leave campaign, raised £2.78m. Its largest supporter was businessman Patrick Barbour, who gave £500,000. Former Conservative Party treasurer Peter Cruddas gave a £350,000 donation and construction mogul Terence Adams handed over £300,000
Rival groups Leave.EU and Grassroots Out, which lost out in the race for the official designation, raised £3.2m and £2m respectively.
Leave.EU received a single £3.2m donation from stockbroker Peter Hargreaves while Grassroots Out was given £1.95m by Better for the Country, a company with links to Leave.EU founder Arron Banks.
In addition, the Leave.EU campaign reported three loans worth a total of £6m.
Other groups campaigning for EU exit have also disclosed money raised above £10,000. They are the Bruges Group (£10,000), WAGTV Ltd (£110,000) and Trade Unionists against the European Union (£22,000).
By virtue of winning the official designations, Britain Stronger in Europe and Vote Leave can spend a maximum of £7m and are entitled to £650,000 in public funding for TV broadcasts, a mailshot and other publicity.
The limits apply to any reportable spending during the regulated campaign period, which began on 15 April and ends at the close of polling on 23 June.
The limits also apply to spending that took place before the regulated period on campaign materials, such as leaflets, which are then used during this period.
The government was criticised by Leave campaigners for spending more than £9m on sending leaflets to all UK households backing EU membership, which happened before the spending limits came into force.
In a separate development, the Electoral Commission has removed 11 groups that were part of the Grassroots Out movement from its official register of campaign groups, meaning they will not be allowed to spend more than £10,000 on campaigning. Groups on the register can spend up to £700,000.
Britain Stronger In Europe said the Commission's ruling was "a damning verdict on Leave campaigns' attempts to circumvent the strict spending rules".
But Conservative MP and co-founder of Grassroots Out Peter Bone said Britain Stronger in Europe's allegations were "absurd, wrong and very disturbing".
He said the 11 groups had been set up in anticipation of Grassroots Out being designated as the official Leave campaign, which it failed to do, and the leaders of the groups had written to the Electoral Commission to say they would not be carrying out any campaigning activities.