Electoral Commission figures show the Tories spent £18.5million from June 9, 2016, to polling day on June 8 last year.
Labour spent £11million and the Liberal Democrats spent £6.8million over the same period. In all, seven parties spent more than £250,000 in the year up to the election.
They included the SNP on £1.6million, Greens on £299,352, Women’s Equality Party £285,662 and Ukip £273,104.
Best for Britain, spent £353,118 on a Remain campaign and National Union of Teachers’ £326,306 on a campaign supporting Labour.
The Commission also announced yesterday that the Tories, Labour and the Green Party faced investigation for submitting spending returns that were missing invoices and for “potentially inaccurate statements of payments made”.
In addition, the Women’s Equality Party was also being investigated for submitting a spending return that was “inconsistent with its dontion reports” for the period.
Best for Britain and the NUT were also under investigation for missing invoices.
Bob Posner, the commission’s director of political finance and regulation, said: “It is vital voters are given an opportunity to see accurate and full reportable data on what parties and campaigners spent money on in order to influence them at last year’s general election.
“This provides transparency in the political finance system and is open for anyone to scrutinise.
“We are investigating possible breaches of the rules.” Meanwhile, the Tories have been warned they face “an all-time low” in the number of their councillors being re-elected in London in the local elections on May 3.
Tory ex-MP Robert Hayward – now Lord Hayward – forecast they could fall below the low of 519 at the 1994 London borough elections.
They won 612 seats in 2014.
He said they could hold on to Wandsworth and Westminster.
He added: “I would be surprised if the Tories do not have an all-time low in the number of their councillors in London this May.”
Elections will be held at councils in England, including London boroughs.