The Conservative Party has been fined a record £70,000 and its treasurer reported to the police for failing to properly declare £275,000 of election spending.
An Electoral Commission investigation found there were "significant failings" in the way spending was reported during the 2015 election campaign and three by-elections in 2014.
It concluded there was a "realistic prospect" the party gained a "financial advantage" from this and that the party's then treasurer Simon Day may have known the spending returns were wrong.
The report comes amid a continuing police investigation into election fraud and whether the party breached strict spending limits in targets seats, including the South Thanet seat contested by Nigel Farage.
The Commission report into the investigation disclosed:
:: In all, the expenses returns for the 2015 election were missing at least £104,765 of payments, £118,125 were incorrectly reported, £52,924 of receipts were not provided
:: No records were kept for amounts invoiced to candidates in three 2014 by-elections.
:: The party failed to declare two-thirds of its £102,483 spending on its Battlebus2015 campaign because of "human error"
:: The Battlebus2015 national campaign resources were in part used to campaign for individual candidates but declared as national Conservative Party spending
:: Spending in South Thanet was attributed to the "national" campaign when some of it should have been included in the Tory candidate's election expenses
:: The commission had to get a High Court order to get information from the Conservative Party to help with the investigation and "caused a delay"
:: Mr Day, now Chief Executive Office of Tory Central Office, was interviewed "under caution" for the investigation and has been reported to the Metropolitan Police.
Despite the fact the commission had to go to court to get the information it needed from the party, Theresa May insisted they had "complied fully" with the investigations. She added that the party would be paying the fine.
Chair of the Electoral Commission Sir John Holmes said the Tories had "undermined voters' confidence in our democratic process".
He also warned he feared fines were no longer a disincentive to failing to play by the rules and were now seen as a "cost of doing business".
SNP MP Pete Wishart described the £70,000 fine as "absolute peanuts" and added "this could be the cash for honours of this Parliament".
When questioned by Sky News Political Correspondent over the fine, Conservative Chairman Patrick McLoughlin grabbed his phone and batted questions away.
A spokesman for the Conservative Campaign Headquarters said it had always taken the view that the "nationally directed battle bus campaign was part of its national return".
It said it would have had no reason not to declare it under national spending because the party was £2m below the threshold.
The amount parties are allowed to spend on campaigning nationally is far, far higher than the £15,000 per constituency that individual candidates are restricted to.
The row goes right to the heart of Downing Street - Theresa May's closest aide, Nick Timothy, was based in South Thanet during the election campaign.
The fine came as it was separately disclosed that file from 12 police forces relating to Conservative election expenses during the 2015 general election have been passed to the Crown Prosecution Service.
South Thanet Conservative MP Craig Mackinlay is understood have been questioned by Kent Police for six hours last week. At least two other MPs are also thought to have been quizzed.
If Conservative MPs are found to have committed any offence, their political opponents could ask for the contests to be rerun.
Up to 24 MPs are understood to be under investigation raising the prospect Mrs May could face a significant number of by-elections at a time when she enjoys a small majority.
UKIP chairman Paul Oakden announced on Thursday that parliamentary candidate nominations for the party for South Thanet have now opened.
Leaked emails, shown to Sky News earlier this week, revealed furious in-fighting in the party with MPs affected accusing party officials of "covering their own backsides" and effectively hanging them out to dry.
A Conservative Party spokesman said: "Political parties of all colours have made reporting mistakes from time to time."
He added: "This is the first time the Conservative Party has been fined for a reporting error. We regret that and will continue to keep our internal processes under review to ensure this does not happen again."