Mark Menzies has quit the Conservative Party and will leave Parliament at the next election after an internal investigation found “a pattern of behaviour that falls below the standards expected of MPs”.

The Fylde MP has been accused of misusing political donations to cover his medical expenses and pay off “bad people” who had locked him in a flat and demanded money for his release.

An investigation into the allegations, denied by Mr Menzies, concluded on Sunday, finding that while there had not been a misuse of party funds, the MP’s behaviour fell below the required standards and may have breached the Nolan principles of public life.

Announcing his resignation, Mr Menzies said: “It has been an enormous privilege representing the people of Fylde since 2010, but due to the pressures on myself and my elderly mother, I have decided to resign from the Conservative Party and will not stand at the forthcoming general election.

“This has been a very difficult week for me and I request that my family’s privacy is respected.”

On Wednesday, The Times reported that Mr Menzies had called his 78-year old former campaign manager, Kate Fieldhouse, at 3.15am one day in December, claiming he was locked in a flat and needed £5,000 as a matter of “life and death”.

The sum, which rose to £6,500, was eventually paid by his office manager from her personal bank account and subsequently reimbursed from funds raised from donors in an account named Fylde Westminster Group, it is alleged.

The Times also reported that £14,000 given by donors for Tory campaign activities was transferred to Mr Menzies’ personal bank accounts and used for private medical expenses.

On Sunday, a party spokesman said the money in question had been signed off by two signatories of the Fylde Westminster Group.

They said: “This body sits outside of the remit of both the Conservative Party and Fylde Conservative Association. Therefore we cannot conclude that there has been a misuse of Conservative Party funds.

“However, we do believe that there has been a pattern of behaviour that falls below the standards expected of MPs and individuals looking after donations to local campaign funds which lie outside the direct jurisdiction of the Conservative Party.”

The spokesman said the Tories would retain individuals on managing such accounts and introduce a whistleblowing hotline.

They added: “Whilst outside the initial scope of this investigation, there has also been a recommendation that the actions of the MP in question have also potentially breached the Nolan principles of public life.

“This is due to the nature of the allegations made, but also the repetitive nature of these separate allegations. These will be reviewed by the Conservative Party’s member governance team.”

Opposition parties have called for the police to investigate the allegations made against Mr Menzies, and Lancashire Police said it was “reviewing the available information” after receiving a complaint, understood to have been made by Labour Party chairwoman Anneliese Dodds.

The Conservatives said they would “share any information with the police if they believe it would be helpful to any investigation they decide to undertake”.

Mr Menzies has been MP for Fylde since 2010, and won a majority of 16,611 at the last election.

After the publication of allegations about his use of funds, he lost the Conservative whip and now sits as an Independent.

He becomes the 101st MP to say he will not fight his seat at the next election.

Labour chairwoman Anneliese Dodds said Mr Menzies had “done the right thing” but “serious questions remain for Rishi Sunak and the Conservative Party”, claiming Tory officials had not spoken to the whistleblower in the case.

She said: “The Conservative Chairman and Chief Whip need to immediately come out of hiding and explain what action they have taken to deal with these shocking allegations.

“And Rishi Sunak needs to explain how he will ever deliver on his promise of professionalism, integrity and accountability and drag his party out of the morass of constant sleaze.”