Electoral Commission looks into Conservative by-election spending
The elections watchdog is widening its investigation into the Conservatives' election spending returns.
Last month it launched an inquiry into Tory general election spending in Thanet South following allegations broadcast by Channel 4 News.
It is now also looking at the reporting of hotel bills at the Newark, Rochester and Strood and Clacton by-elections.
The Conservative Party said all spending had been "correctly recorded in accordance with the law".
Strict limits apply to the amount parties and individual candidates can spend fighting elections.
Channel 4 News alleged the Conservative Party's candidate spending returns for the three by-elections were incomplete.
The commission does not have the power to investigate candidate spending, which is the responsibility of the police.
But it can investigate whether the hotel bills were reported in the overall returns for the 2014 European Parliament election and May's general election.
If the Conservative Party did include the hotel bills in its national spending returns it would be up to the police to investigate whether the party should have instead included the bills in the candidate spending returns.
The Commission can sanction a breach of the rules with a fine of up to £20,000.
The Conservatives said: "We will of course cooperate with the Electoral Commission," adding: "All spending has been correctly recorded in accordance with the law."