Channel 4 News has obtained evidence of tens of thousands of pounds of spending by the Conservatives during key by-election campaigns which appear not to have been declared.
Hundreds of pages of receipts obtained by this programme seem to show undeclared expenditure by the party in three crucial parliamentary by-election campaigns in 2014.
The documents appear to reveal a pattern of undisclosed spending and link directly to Conservative HQ and senior figures within the party.
Campaign spending in each by-election is subject to a legal limit of £100,000 to ensure fairness, so contests are not skewed in favour of richer political parties.
If all the receipts had been declared, the party would appear to have flouted spending limits in all three by-elections in Newark, Clacton, and Rochester & Strood during 2014.
All three by-elections were seen as crucial battles to halt the then-growing popularity of UKIP, which was increasingly threatening to steal support from the Tories.
Our investigation obtained copies of receipts for six hotels across the three by-election areas showing stays by large numbers of Conservative campaign workers, including senior party figures.
In all, the hotel bills show 1,401 nights of accommodation, with the total cost of rooms, food, and additional spending amounting to £113,030.63 across the three campaigns.
Of this 1,182 nights with costs of £94,112.19 fell within strict campaigning time limits, known as regulated periods, where spending must be declared by law.
This includes £56,866.75 undeclared hotel bills in Rochester, which would have taken them £53,659.83 over the £100,000 spending limit; £26,786.14 in Clacton, which would take them £10,835.36 over the limit; and £10,459.30 in Newark, which would mean a £6,650.28 overspend.
Under the name "Mr Conservatives" In each of the three by-election campaigns, we found the same pattern with two hotels booked for Conservative campaign workers -- one upmarket for senior staff, and one budget Premier Inn for more junior campaign members.
Receipts for the all of the six hotels do not appear to have been declared in the material submitted to the Electoral Commission.
The vast majority of these hotel stays - 734 in the regulated periods - were booked under the home address of Marion Little, a senior staffer at Conservative Central Office awarded an OBE in this year's New Year's Honours List for her work during the campaigns.
Some rooms were booked under the name "Mr Conservatives".
Channel 4 News has obtained evidence of tens of thousands of pounds of spending by the Conservatives during key by-election campaigns which appear not to have been declared.Hundreds of pages of receipts obtained by this programme seem to show undecla