The Conservative Party is facing questions about failing to declare tens of thousands of pounds of spending during three by-election campaigns.
Channel 4 News said it had obtained hundreds of pages of receipts which suggested the party broke electoral rules in the 2014 contests in Newark, Clacton and Rochester & Strood.
But a party spokeswoman said: "All by-election spending has been correctly recorded in accordance with the law."
The Tories defeated Ukip in Newark, but went on to lose the other two by-elections.
Campaign spending in by-elections is limited to £100,000 for each party. Channel 4 News said it had uncovered £56,866.75 of undeclared hotel bills in Rochester, which would have taken the party £53,659.83 over the £100,000 spending limit.
It found bills totalling £26,786.14 in Clacton, which would have taken the party £10,835.36 over the limit, and bills totalling £10,459.30 in Newark, which would meant a £6,650.28 overspend.
Ukip's Clacton MP Douglas Carswell told the programme: "It's shocking ... It's simply intolerable. You have to have a law to make sure you don't have unfair advantage.
"We need to look at whether the Electoral Commission should have the right powers. I will certainly be raising it (with the commission)."