The grassroots group that propelled Jeremy Corbyn into the Labour leadership and campaigned for him in last year's election is facing fresh questions over potential breaches of rules governing campaign donations.

Momentum, which is already being investigated by the elections watchdog over allegations relating to the accuracy of its financial returns in 2017, declared no cash or "in kind" donations worth more than £500, despite organising activist training sessions led by professional campaigners from the US.

A string of former members of the campaign to make Bernie Sanders the Democratic nominee for president led around 20 activist training sessions for Momentum between April and June last year. They included Claire Sandberg, Mr Sanders' director of distributed organising, and Jeremy Parkin, a field organiser for the Bernie 2016 campaign, who has described his role as "training volunteers and local Momentum chapters in canvassing and persuasion techniques" over a two-month stint.

None of the individuals was paid, but Momentum said at the time that it was "providing reasonable expenses". Electoral Commission rules state that the total value of any money, goods or services worth more than £500 and provided for free to "non-party campaigners" such as Momentum, must be declared following an election campaign.