It follows reports that Tories failed properly to declare spending in seats such as Dudley South following last year’s general election .
Investigations by the Daily Mirror and Channel 4 News found that the party didn't include the cost of transporting volunteers across the country on its “battle bus”, and putting them up in hotels, in local candidate spending returns.
The spending includes more than £38,000 putting up volunteers in hotels across the country.
And if the spending is considered to be local campaigning, rather than part of the national campaign, it means the amount spent in 24 constituencies across the country would be have exceeded the legal spending limits set out by law.
Constituencies where Conservative activists campaigned as part of the battle bus tour include Dudley South, where Tory Mike Wood was elected.
Conservatives declared spending of £11,452 in the constituency, where the spending cap was £12,296. But if a share of the battle bus spending is added to the total, it comes to £13,498 - £1,202 over the spending cap, according to Channel 4 News.
Dudley South is a marginal seat which Labour held from 1997 to 2010. Conservatives won the seat in 2010 and held it in 2015 with a majority of 4,270.
Channel 4 News has also reported that the Conservatives failed to include the cost of the battle bus in its local candidate spending return for Wolverhampton South West, another marginal seat, which Labour won from the Conservatives in last year’s election.
Detective Chief Inspector Ed Foster, from West Midlands Police, said: “We have received an allegation of improper electoral campaign spending returns in the West Midlands area and are reviewing whether any offences have been committed.
“We will be applying for an extension on time to investigate and continue to liaise closely with the National Police Chiefs Council and the Electoral Commission.”
The Conservative Party has acknowledged that some accommodation costs for the activists were not properly registered due to an “administrative error”, but insisted the bus tour was part of the national campaign organised by Conservative campaign headquarters and did not have to fall within individual constituency spending limits.
Staffordshire, Warwickshire and West Mercia police are also looking into Conservative election spending.