The elections watchdog has announced it is extending the scope of an investigation into Conservative Party campaign spending.
The Electoral Commission last month launched an inquiry into Tory general election spending in the Kent seat of Thanet South in 2015, following allegations broadcast by Channel 4 News.
The investigation has now been broadened to cover claims also broadcast by C4N relating to spending on hotel bills for activists during the 2014 by-elections in Newark, Clacton and Rochester & Strood.
The investigation will now look into whether the bills counted as campaign spending incurred by the Conservative Party which should have been reported to the Commission.
And it will consider whether spending on the by-elections was reported correctly as part of the party's spending returns for the last European Parliament election and the general election, in accordance with regulations.
Parties are required to report any campaign spending during the so-called "regulated period" before a national election, during which expenditure is capped.
The Newark by-election fell within the regulated period for the 2014 European poll, while the other two by-elections took place during the general election regulated period.
The Commission has no powers to investigate or sanction candidate spending offences, which are matters for the police.
The announcement came after Labour MPs and defeated candidates called for a police investigation into the Conservatives' use of battle buses to ferry activists to campaign in marginal seats.
The Conservative Party insisted that spending on the campaign was in accordance with election laws.
A Daily Mirror investigation found the controversial RoadTrip events helped 24 Tory MPs win their local contests but none declared it in their campaign budget.