LONDON — Facebook and Twitter have promised to cooperate with another set of investigations into suspicions of Russian meddling in elections — this time in Britain, over the bitterly divisive referendum last year in which the country voted to leave the European Union.

More than 150,000 Russian-language Twitter accounts posted tens of thousands of messages in English urging support for withdrawal, known as Brexit, in the days before the vote on June 23, researchers have found, drawing parallels with Russian social media activity around the United States presidential election later that year. Facebook has acknowledged that more than 126 million users may have seen inflammatory political ads bought by a Russian company, the Internet Research Agency, during the American campaign.

In a letter to the chairman of a British parliamentary committee, released on Tuesday, Facebook promised to comply with a request for data “by the second week of December,” adding that its response would also be sent to the country’s official election watchdog, the Electoral Commission. Twitter made a similar promise to release material “in the coming weeks.”

The committee chairman, Damian Collins, a Conservative lawmaker who leads the House of Commons Digital, Culture, Media and Sport Select Committee, had written to Facebook’s founder and chief executive, Mark Zuckerberg, requesting that the social network share material similar to that given to congressional committees investigating Russian election meddling in the United States.