Police are looking into Conservative claims that Labour’s candidate in a crunch election on Thursday wrongly claimed to live in the area to stand.
West Midlands Police said it was “assessing” the allegations against Richard Parker.
A spokesman for Mr Parker described the claims as “shameful political game-playing”.
Labour and the Tories are thought to be neck and neck in the race, which could prove crucial to Rishi Sunak’s future.
Current Conservative mayor Andy Street is hopeful of winning.
But a loss could encourage Tory rebels in their campaign against the prime minister.
West Midlands Police said in a statement: “We can confirm that we have received an allegation and are currently assessing it.”
The allegations centre on claims Mr Parker may have breached electoral law.
Gary Sambrook, the Tory MP for Birmingham Northfield, has written to the police over claims Labour’s candidate may have broken residency rules to get on the ballot paper.
Central to the row is the claim that on official nomination papers Mr Parker declared a rented flat in Birmingham as his address, but his main property is said to be outside the West Midlands Combined Authority area he wants to run.
Candidates have to be registered to vote and ”during the whole of the preceding 12 months resided in” in the area to stand.
Over the same period candidates have to say that they have “occupied as owner or tenant of the following land or other premises” in the area.
The Conservatives are predicted to lose up to 500 council seats in the local elections.
The party is also facing two key re-election battles, for the mayor of the West Midlands and the Tees Valley’s Ben Houchen.
Polls suggest both could be very close contests.
Already senior Tories are moving in an attempt to shore up Mr Sunak. Home secretary James Cleverly has warned Conservative MPs considering a leadership challenge to after next week’s local elections that it would be a “catastrophic idea”.
William Turner is a seasoned U.K. correspondent with a deep understanding of domestic affairs. With a passion for British politics and culture, he provides insightful analysis and comprehensive coverage of events within the United Kingdom.