Three top Labour MPs are facing calls to return hundreds of thousands of pounds donated to them by a single company.
Left-wing campaign group Momentum has demanded Wes Streeting, Yvette Cooper and Dan Jarvis return more than £345,000 given to them by MPM Connect Ltd - donations it has branded "dark money".
As part of the Westminster Accounts project, which has collated MPs' second jobs earnings and donations in one database, the company has been highlighted as being the third largest donor to MPs of any party this parliament, since December 2019.
The company has no staff or website and is registered at an office in Hertfordshire where the secretary says she has never heard of them.
Ms Cooper, the shadow home secretary, has received £184,317, former mayor of South Yorkshire Mr Jarvis £100,000 and Mr Streeting, the shadow health secretary, £60,900.
They are the only MPs MPM Connect has donated to.
Sky News has approached the three MPs for comment in response to Momentum.
The left-wing group is also calling on Sir Keir Starmer to adopt stronger policies to "clean up politics" and "get our own house in order".
'Dark money donations'
Kate Dove, Momentum co-chair, said: "It's no surprise to see Tory MPs dominating the Westminster Accounts list - everyone knows the Conservative Party is up to its neck in sleaze.
"What is shocking, however, is that Labour MPs are joining them in accepting dark money donations - even senior members of the shadow cabinet like Wes Streeting and Yvette Cooper."
She added that Momentum is calling on Sir Keir to ban "the use of shell companies to funnel dark money into politics" and "commit to kicking corporate interests out of political financing together".
MPM Connect's entry in the Companies House register lists two directors - recruitment mogul Peter Hearn and Simon Murphy, the entrepreneur behind the redevelopment of Battersea Power Station.
The company's accounts do not disclose where it receives its funding, what it does or why it donates so heavily.
When Sky News went to the office in Hertfordshire, where the company is registered, the receptionist in the building denied any knowledge of MPM Connect.
She told Sky News she did not recognise the names of the two directors.
"We'd rather not speak to you," she said, before closing the door.
Momentum was founded in 2015 following the election of Mr Corbyn as leader of the Labour Party, with the stated aim of transforming the Labour Party.
It was hailed as an effective grassroots force by supporters, while detractors criticised it for being reminiscent of Militant - the Trotskyist group which aimed to push the party to the left until it was proscribed in the 1980s.
Earlier this week, Ms Cooper told Sky News: "MPM Connect is an investment company in the employment sector owned by Peter Hearn, a UK businessman and long standing Labour supporter, and as has been reported many times, Mr Hearn kindly donates money to fund Yvette's office staff and has done so for many years.
"It has all been fully declared and compliant with all the rules. Like lots of companies, MPM Connect is registered to their accountants' office, as is made very clear in its Companies House entry."
Mr Streeting's spokesman said: "Peter kindly funds staff for Wes's office. Wes is proud to have successful business people supporting the Labour Party in our campaign to win the next election and give Britain the fresh start it deserves. This has been declared in the proper way in accordance with the rules."
Mr Jarvis said all his donations support his work as an MP.