British Prime Minister Theresa May leaves 10 Downing Street in London on Nov. 1, 2017. (Toby Melville/Reuters)

New investigations released this week suggest the Russians meddled in Britain's historic referendum last year to leave the European Union, placing an already weakened Prime Minister Theresa May in a most awkward position — just when she needs to be her strongest in Brexit negotiations.

The evidence that the Russians, with possible support from the Kremlin, bombarded British targets with social media tweets and posts was splashed on the nightly news and front pages in Britain.

Even so, the prime minister and her office stressed that Russian propaganda had "no direct successful influence" on the Brexit vote. 

Critics of May say an admission that Russia tried to dupe British voters could raise questions about the Conservative Party's mandate to extricate Britain from the European Union.

The specter of Russian meddling also stokes internal discord among the Tories, who are divided between those who demand May secure a total, complete exit from Europe vs. those advocating a "soft Brexit." There is also a small but stubborn rearguard of Conservatives who say Brexit is a historic blunder.

Labour Party politicians blame the prime minister for what appears to be a muddled message on Russian meddling. 

On Monday, May gave a tough and well-received speech before a group of business leaders in London, accusing President Vladimir Putin's Russia of attempts to "undermine free societies" and "sow discord" in the West by "deploying its state-run media organizations to plant fake stories."

"So I have a very simple message for Russia," May warned. "We know what you are doing. And you will not succeed."

The speech was widely contrasted with President Trump's remarks two days earlier, in which he appeared to defend Putin. "He said he absolutely did not meddle in our election," Trump said. "He did not do what they are saying he did."

British intelligence agencies have been gathering information on Russian active measures and disinformation campaigns before and after the Brexit referendum.

Russia denies meddling in the Brexit vote and has suggested it is being used as a scapegoat.

"British society is currently not going through its finest hour due to the ongoing process of #brexit," Russia's Foreign Ministry tweeted. "It is understandable that an external enemy is direly needed to distract public attention for which role Russia has been chosen. It is deeply regrettable."

Asked this month about possible Russian interference in British elections, Foreign Secretary Boris Johnson replied that "Nyet," and added there was "not a sausage" of evidence.

In Parliament on Wednesday during the rough-and-tumble Prime Minister's Questions session, the Labour Party's Mary Creagh pressed May: "Has the foreign secretary been kept in the dark on the intelligence, has he not read it, or is he willfully blind? And will [May] now stop dragging her feet and set up the Intelligence and Security Committee to look urgently into the Kremlin's attempts to undermine our democracy?"

May promised that the parliamentary committee would begin its work that day. But on the question of what Johnson knew or did not know, May surprised some when she stressed that in her speech on Russian bots and trolls, she had not meant to imply that Britain was the target.

"If they care to look at the speech on Monday," May said, "they will see that the examples I gave were not in the U.K."

This prompted head-scratching. On Thursday in Parliament, Labour Party lawmaker Ben Bradshaw said May's ministers have gone out of their way to avoid talking about Russian interference in the Brexit vote — until now. He suggested that the mounting evidence from intelligence agencies and independent researchers may be forcing her hand.

In an interview with The Washington Post, Bradshaw said he was unsure why May was downplaying Russian interference in Britain.

"I don't know whether they are doing this because they would be concerned to do anything that might cast doubt on the legitimacy of the very close referendum result," Bradshaw said. "Or they may be nervous about doing anything that could embarrass the U.S. president, particularly when they seem to be pinning all of their hopes after the disaster of Brexit on some fantasy trade deal from the White House."

Analysts say May is responding to her domestic political realities. "I don't think she wants people to run away with the idea that somehow Brexit was caused by the Russian Federation," said Tim Bale, a professor of politics at London's Queen Mary University. 

Bale said the issue is not black-and-white. Yes, he said, there is growing concern regarding Russian meddling, but not in the sense that it made any measurable difference in the Brexit vote. "Because there is mounting evidence from various European democracies and the U.S. of interference in their democratic processes, and that's not something any government or state in the West is prepared to tolerate," he said.

In a speech Wednesday, Ciaran Martin, chief executive of the National Cyber Security Center, accused Russian hackers of attacking the British media, telecommunications and energy sectors. 

"That is clearly a cause for concern — Russia is seeking to undermine the international system," he said. 

Researchers at Swansea University in Wales and the University of California at Berkeley found that more than 150,000 Twitter accounts with ties to Russia tweeted about Brexit in the days before the Brexit vote. 

Their findings, which are part of a bigger research project and are not yet published, were first reported in the Times of London.

Tho Pham, one of the researchers at Swansea, said that the accounts they analyzed — which claimed Russian as their language when they were set up but tweeted in English — posted a mixture of pro-"leave" and pro-"remain" messages regarding Brexit. Commentators have said that the goal may simply have been to sow discord and division in society. 

A separate study by researchers at the University of Edinburgh analyzed the 2,752 accounts that Twitter handed over to the U.S. Congress. The accounts, now deactivated, were linked to the Internet Research Agency, a St. Petersburg troll farm with ties to the Russian government. 

The researchers found that more than 400 of those accounts posted divisive messages about the Brexit referendum, the majority after the polls closed. 

Clare Llewellyn and Laura Cram, the academics behind the study, wrote that the findings "provide the first hard evidence that users identified by Twitter as having Russian links and as seeking to influence the US election, were also actively tweeting on Brexit-related issues."

But they also called for caution in trying to determine the influence of the messages on the Brexit vote, in part because of limited data access from social media companies such as Twitter.

"To establish the extent to which the Brexit debate, or indeed the UK general election, were influenced by such users we need an equivalent list of users seeking to target these specific events and a complete data set," they said. 

The Conservative lawmaker Damian Collins agrees. He is pressing Twitter and Facebook to release posts about British politics from Russian-linked accounts. 

"The public have a right to know if foreign powers were acting to undermine the democratic process in this country," he said.

Collins chairs a parliamentary committee that in January launched an investigation into "fake news." The committee intends to travel to Washington early next year to gather evidence from U.S. tech giants. 

Collins has called on Twitter to hand over a list similar to the one given to Congress. Some of those accounts were posting messages about Britain, as well. 

One of the accounts was a user called @SouthLoneStar, which posted a photo of a woman in a headscarf walking on Westminster Bridge. The tweeter wrongly said that the woman was ignoring victims of a terrorist attack. 

The tweeter — described as a "proud Texan" — wrote "Muslim woman pays no mind to the terror attack, casually walks by a dying man while checking phone #PrayForLondon #Westminster #BanIslam."