Police criticized after stalling for 2 months on whether to investigate Boris Johnson's campaigning with a taxpayer-funded jet

• Police said in August that they were 'reviewing correspondence' on the Hartlepool by-election.
• It followed Insider reporting on apparent undeclared spending on a jet flight for Boris Johnson.

Two months after police said they were contemplating whether to investigate Prime Minister Boris Johnson's apparent use of a taxpayer-funded jet in the Hartlepool by-election, officers are yet to decide what to do.

The delay prompted criticism from opposition politicians and transparency campaigners, who urged officers to open a formal investigation.

Insider revealed in August that the Conservatives' spending return in the Hartlepool Parliamentary by-election, which they won, claimed no expenditure on transport.

The statement was despite Johnson flying to a nearby airport, where he made an official visit and then campaign alongside the Tory candidate in the election, Jill Mortimer, who went on to win.

The Ministerial Code says "where a visit is for a mixture of political and official engagements, it is important that the department and the Party each meet a proper proportion of the actual cost."

Insider subsequently acquired a second Conservative Party spending return for the Tees Valley mayoral contest in which the party also says it spent nothing on transport, despite Johnson also appearing and campaigning with the Tory candidate for the mayoralty.

The Conservative Party has repeatedly denied any wrongdoing. Both party and Downing Street spokespeople have insisted the jet was used solely for government business.

Cleveland Police's role in any investigation would be to do with whether the spending returns were accurate, which is a police matter under the Representation of the People Act 1983.

The Prime Minister's Independent Adviser on Ministers' Interests has already declined to investigate whether the incident broke the Ministerial Code.

After two months without a new public statement, campaigners and politicians told Insider that Cleveland Police ought to provide an explanation.

The deputy leader of the Liberal Democrats, Daisy Cooper MP, said an investigation must be opened "without delay".

"If the Prime Minister has done nothing wrong then he should welcome an investigation."

"The public deserve to know whether or not the PM used a tax-funded jet to do election campaigning and whether the Conservative Party failed to declare or pay for it.

"This government has played fast and loose with the Ministerial Code before, taking voters for granted; an investigation into these allegations needs to happen without further delay."

Naomi Smith, chief executive of the internationalist campaign group Best for Britain, told Insider: "The Conservatives cannot be allowed to use taxpayer money like a campaign slush fund and two months on from saying they are reviewing the matter, Cleveland Police are yet to disclose their findings on what appears to be a clear breach of electoral law.

"Sunlight is the best disinfectant and this is an issue that cuts to the core of transparency and fairness of our elections. If the police are not taking any action, they must explain why."