Labour has called for an investigation into £75,000 of donations to Tory leadership contender Robert Jenrick, saying it had “serious concerns” about the money’s ultimate origin.

Mr Jenrick, the frontrunner for the Conservative leadership, received three donations of £25,000 in July from The Spott Fitness, a fitness coaching app provider.

As first reported by the Tortoise news website, the company’s latest accounts show it has no employees, has never made a profit and has more than £300,000 of debts, and in January it registered a loan from Centrovalli, a company registered in the British Virgin Islands.

The ownership of companies registered in the British Virgin Islands is not made public, leading Labour to question where the money donated to Mr Jenrick ultimately came from.

Party chairwoman Ellie Reeves said in a letter to the Electoral Commission: “Donations to MPs must come from sources registered in the UK. It is clear that Mr Jenrick has serious questions to answer about the origin of these funds and their legality.”

A source close to Mr Jenrick’s campaign dismissed Labour’s request as “nonsense”, saying it served to “prove who Labour fear the most”.

Speaking to Sky News earlier on Sunday, Mr Jenrick insisted no laws had been broken.

He said: “As I understand it, this is a fitness company that operates in the UK.

“It’s a perfectly legal and valid donation under British law and we’ve set it out in the public domain in the way that one does with donations.”

The questions come amid rows about senior Labour figures accepting donations from Lord Waheed Alli, although Mr Jenrick stressed he was not criticising the donations but what he called Labour’s “rank hypocrisy”.

He said: “They spent years complaining about other political parties and then they’ve chosen to take off donors and cronies and to give passes to Number 10 in response.”

Britain’s laws on political donations have long been criticised by transparency campaigners as too weak to prevent anonymous money from impermissible donors entering the system.

Earlier this year, Transparency International UK told a parliamentary committee there was “a growing body of evidence suggesting it is far too easy for money of unknown or questionable provenance to enter our political system”.

It pointed to the “low bar” for foreign companies to be allowed to donate money to UK politicians – firms only have to be “carrying on business in the UK” and there is no requirement for donations to be generated from UK profits – and claimed existing rules had proven “almost unenforceable in practice”.