The chief constable of the police force examining claims against Angela Rayner has defended the decision to investigate the Labour party deputy leader, vowing it would be done “fairly and impartially” and would establish whether “culpability is proven or otherwise”. Stephen Watson, who leads Greater Manchester police (GMP), gave his most extensive public comments so far on the investigation, which was triggered by a complaint from a senior Conservative. Watson’s interview with the Guardian was given to mark his third anniversary leading the force, becoming chief constable when it was failing so badly it was in special measures , and is credited with dragging it out of crisis.

The decision by his force to investigate Rayner – who denies any wrongdoing – brings policing and politics potentially into conflict again. GMP first said it would not investigate claims against Rayner over whether the house she once lived in Stockport from 2010 was her main residence and whether she thus may have avoided tax when she sold it. The Labour leader, Sir Keir Starmer, has called claims his deputy lied about her primary residence to avoid tax a “smear” .

Watson told the Guardian it was a letter from the Conservative deputy chair, James Daly, that led to his force’s dramatic reversal. He said: “We received an initial piece of information, which was assessed and led us to a certain conclusion. “We then subsequently received a letter, it’s a matter of record, from James .