Labour has called on Parliament's sleaze watchdog to investigate Reform UK’s Richard Tice over allegations of another unregistered donation to the party.

Labour chair Bridget Phillipson has written to Daniel Greenberg, Parliament’s commissioner for standards, to ask him to probe “a possible failure… to register a financial or material benefit within the period required” strict House of Commons rules.

In her letter Ms Phillipson points to reports that Fiona Cottrell, the mother of Nigel Farage’s close associate George Cottrell, who has been dubbed “posh George”, transferred more than £1m to Britain Means Business Ltd, a company owned and controlled by Mr Tice, after receiving $1.4 million from her son.

On the same day, Britain Means Business donated £500,000 to Reform UK and Reform paid £200,000 towards the debt it owed to Tisun Investments Ltd, another company ultimately controlled by Mr Tice.

“The central question is whether Mr Tice received a registrable financial or material benefit that should have been entered in his Register of Members' Financial Interests, and whether the source of that benefit was properly disclosed,” Ms Phillipson said.

It comes as Reform UK has been reported to the police over allegations it breached political donation rules by having polling paid for by an overseas company.

Mr Greenberg is already probing claims Mr Farage failed to register a £5m “gift” from a cryptobillionaire.

Ms Phillipson’s letter continued that “if any of the money supplied by Fiona Cottrell was used, directly or indirectly, to convert Reform's debt to Tisun into donations, then there is a serious question whether Mr Tice - or a company he controls - received a registrable benefit linked to his political activities”.

At the very least, Labour argues, his register of MPs interests entry “would need to make clear the source, the intermediary and the practical effect of the transaction”.

Mr Greenberg is already probing the £5m donation, which Mr Farage insists he did not need to declare because it was a personal gift to him from Mr Harborne.

The Reform UK leader triggered a by-election last month in a bid to, as he put it, let the voters in his constituency decide the issue. However, if the findings of the investigation go against him, he could still face another by-election and potentially be ejected from Parliament.

Mr Tice told the call from Labour was “more twaddle by a desperate Bridget Phillipson, who has herself just breached the rules by publicising her complaint. I am confident that the rules have been fully complied with”.

Reform and Mr Greenberg have been approached for comment.