The Electoral Commission has begun making inquiries into a donation to the Conservative Party made by an American banker through her British firm.

The watchdog, which rejected requests for an investigation into George Osborne, confirmed that it was examining a payment from an investment firm run by the US-born Robin Saunders, days after a personal cheque from her for the same amount was rejected.

Ms Saunders said that she used the wrong chequebook to buy a ticket to the party’s annual black-and-white fundraising ball in February. The Conservatives concede a “clerical” error, but insisted that her Clearbrook Capital is a UK-trading firm and therefore a permissible donor. The firm wished to take a table at a Tory party function, they added.

But Labour says that the £6,600 payment is “flouting” the ban on foreign donations. In a letter to the Electoral Commission, John Mann, Labour MP for Bassetlaw, said that, under legislation passed in 2000, assisting in the “diverting” of donations was illegal.

Last night Labour raised questions over a second donation by a US citizen. The commission recorded an attempted £4,000 donation by the US-born Nancy Broadbent to Andrew Mitchell, the Shadow International Development Secretary, on November 1 last year. It was returned by Mr Mitchell’s office on November 15 because it was impermissible.

The commission register then records Dominic Casserley, her husband and a partner at McKinsey & Co, giving the same sum to Mr Mitchell 13 days later. Labour called this “another example of foreign money being accepted”.

Mr Casserley insists that he was responsible for the donation, but asked his wife to write the initial cheque as he was out of the country on business.

It is understood that Mr Casserley is a long-term friend of Mr Mitchell from Cambridge, and had given money the previous year, as well as donating £10,000 to David Cameron. His wife had never given money before.

A McKinsey representative said: “This donation was made in a personal capacity and done in accordance with Electoral Commission rules governing such donations.”

The Conservatives deny any wrongdoing, saying that they always comply with the law. Mr Mann added: “The Electoral Commission really needs to investigate what appears to be a systemic problem surrounding Tory party fundraising.”

Times Recommends

• How I lost the gift of joy for a while
• Why Ann Widdecombe has got it wrong
• Alas, it’s the end of the road for petrolheads