The Conservative Party has been fined £70,000 by the Electoral Commission for failing to report its election expenses accurately.
The watchdog said candidates had failed to declare spending on a "battlebus" which took volunteers to campaign in marginal seats in the North, the Midlands and the South West during the 2015 general election campaign.
A report setting out the Electoral Commission's findings does not name the constituencies involved. However, Tory activists on the bus reported their visits on Twitter as the election campaign took place, and revealed that they visited seats including Dudley South, Nuneaton and Cannock Chase.
These are marginal Midlands seats which the Conservatives were defending and feared losing to Labour . They were won by Tory MPs Mike Wood, in Dudley South; Marcus Jones, in Nuneaton, and Amanda Milling in Cannock Chase.
The Conservative Party recorded the cost of the bus, known as Battlebus2015, as general party expenditure rather than as part of campaigns to elect the individual candidates.
But the Electoral Commission found this was incorrect. It said: "Coaches of activists were transported to marginal constituencies to campaign alongside or in close proximity to local campaigners.
"In the Commission’s view, there was a clear and inherent risk that activists might engage in candidate campaigning. Further, it is apparent that candidate campaigning did take place during the Battlebus2015 campaign."
The total recorded cost of the battlebus was £38,996.06, including the volunteers’ accommodation, coach travel and subsistence.
According to the Electoral Commission, the Conservative Party also failed to maintain records explaining the amounts it invoiced to candidates in three 2014 by-elections, for work on their campaigns which meant the accuracy of the amounts could not be verified.
The Conservative Party’s 2015 UK Parliamentary general election spending return was missing payments worth at least £104,765.
And the party did not include the required invoices or receipts for 81 payments to the value of £52,924.
Among the seats investigated by the Commission was South Thanet, where Tory Craig Mackinlay narrowly beat former Ukip leader Nigel Farage.
In the report on its investigation, the Commission said: "The advantage obtained by party by its actions; the inclusion in the party return of what in the Commission's view should have been reported as candidate spending meant that there was a realistic prospect that this enabled its candidates to gain a financial advantage over opponents.
"In this respect the Commission noted that the Battlebus2015 campaign visited target constituencies and that South Thanet was also a key priority for the party."
A Conservative Party spokesman said: “CCHQ [Conservative Campaign Headquarters] has always taken the view that its nationally directed battlebus campaign – a highly-publicised and visible activity with national branding – was part of its national return, and it would have no reason not to declare it as such, given that the Party was £2 million below the national spending threshold.
“The Electoral Commission report makes clear that our interpretation of the guidance was correct, stating: ‘The Commission has found no evidence to suggest that the Party had funded the Battlebus2015 campaign with the intention that it would promote or procure the electoral success of candidates.
“MPs in constituencies visited by the battlebus would have no reason to consider whether it should be included in their local return – they were directed that the bus would be visiting as part of CCHQ’s national spending."
A dozen police forces have passed files to the Crown Prosecution Service over allegations that up to 20 Conservative MPs broke local spending limits at the last general election because the cost of battlebus visits was wrongly considered to be national expenditure.
They include Avon and Somerset, Cumbria, Derbyshire, Devon and Cornwall, Gloucestershire, Greater Manchester, Lincolnshire, London, Northamptonshire, Nottinghamshire, Staffordshire and West Yorkshire. Once it has received a file, the CPS will decide whether to charge anyone.