LONDON — Parliament is asking Facebook, Twitter and other internet companies for information about Russian efforts to use social media to influence the British vote to leave the European Union, expanding on several parallel American congressional investigations around the election of President Trump.

The British inquiry adds to the mounting pressure for more disclosure from the internet giants, which have already acknowledged broad Russian efforts to influence national elections in both the United States and France.

Any indication of Russian meddling in the British vote last year to leave the European Union could raise doubts about the integrity of the outcome, further complicating the already vexed domestic politics surrounding the departure, commonly called Brexit.

The Kremlin, which has long viewed the European Union as a threat, had a strong interest in promoting the groups that favored Britain’s leaving the bloc. And the most visible tools of Russian propaganda, like the English-language broadcaster RT and the news agency Sputnik, heavily supported the initiative. The leave camp won with 52 percent of ballots cast, about 1.3 million more votes than the remain side drew.