The Labour shadow minister for media, Christian Matheson, said that he would pass on McDonnell’s concerns to the Electoral Commission.
“I can say that the commission is aware of occasions and delegations in the past where people who might not have been expected to have a certain amount of resource were suddenly able to spend that resource.
“They assure me that they monitor the activity of non-party campaigners, and where there is evidence that the law has not been followed it will consider the matter in line with its enforcement policy.”
Electoral transparency campaigners have warned that anonymous campaign spending, through third-party campaigners, risks damaging British democracy.
“Voters are being left in the dark about who is funding our politics, by out-of-date campaign rules and lax enforcement. There is a serious and sustained lack of transparency about who is steering our political debate,” Willie Sullivan, senior director at the Electoral Reform Society, told openDemocracy last month.
“Despite repeated calls for reform – from parliamentary committees to civil society groups and voters – almost no action has been taken. Ministers must close the loopholes that allow millions to be spent on political influencing with almost no scrutiny.”
The Electoral Commission has said that it is the legal responsibility of campaigners to ensure that their donations are declared fully.
The elections watchdog has also said that it monitors activity and would consider action where there is evidence that rules have not been followed.