He said there was a pattern of behaviour of Russian organisations seeking out opportunities to create division, unrest and instability in the West.
"Foreign organisations have the ability to manipulate social media platforms to target voters abroad," he told AFP.
"This is seriously-organised buildings of hundreds of people engaged in propagating every day fake news through social media."
He said it was "terrifying" how cheap and easy it was for them to reach millions of people.
"It is one of the biggest threats our democracies face and we have to be serious about combatting it," Collins added.
May's spokesman insisted: "There has been no evidence of successful interference in our electoral processes."
Researchers at the University of Edinburgh, who examined 2,752 accounts suspended by Twitter in the United States, found 419 were operating from the Russian Internet Research Agency and attempting to influence British politics, The Guardian reported.
Professor Laura Cram, the university's neuropolitics research director, told the newspaper they tweeted about Brexit 3,468 times -- mostly after the June 23 referendum.
The content overall was "quite chaotic and it seems to be aimed at wider disruption. There's not an absolutely clear thrust. We pick up a lot on refugees and immigration", she said.
Meanwhile researchers at Swansea University in Wales and the University of California, Berkeley, have found more than 150,000 Russian-based Twitter accounts which may have influenced the Brexit referendum.
The social media accounts switched their attention to EU membership in the run-up to the referendum, 2016, according to research outlined in The Times newspaper.
Many of the accounts were fully-automated "bot" profiles which posted hundreds of tweets daily, or "cyborg" accounts which were partially run by people, the newspaper said.
The majority of the posts were pro-Brexit, while some supported remaining in the European Union.
Meanwhile it was revealed that a tweet which caused a furore after the Westminster terror attack in March originally came from a trolling agency account which, according to evidence before the US Congress, is backed by the Russian government.
The tweet showing a picture of a woman in a headscarf walking next to a victim, with the words: "Muslim woman pays no mind to the terror attack, casually walks by a dying man while checking phone".—AFP