LONDON — An opposition lawmaker called on the British government on Thursday to investigate suspicions of Russian interference in Britain’s referendum on quitting the European Union, adding that there were “questions” about Arron Banks, one of the chief financial backers of the leave campaign.

Speaking in Parliament, the lawmaker, Ben Bradshaw of the Labour Party, said there was “widespread concern over foreign, and particularly Russian, interference in Western democracies.” He asked for assurances that “all the resources spent in the referendum campaign were from permissible sources.”

Mr. Bradshaw’s intervention follows the publication of what he called “very worrying” reports “on the role of dark money” in the plebiscite, which was held in June 2016 and in which 52 percent of those who voted opted to leave.

Those who opposed Britain’s withdrawal, or Brexit, argued during the referendum that it would delight Moscow by weakening the European Union, which placed sanctions on Russia. But no links to Russian money have been proved.