British Prime Minister Theresa May is under pressure to return millions of dollars given by Russian oligarchs and their lobbyists to her ruling Conservative party. One of the biggest donors is the wife of a former Russian deputy finance minister, once nicknamed “Putin’s banker.”

When May took office 18 months ago she promised that Britain’s Conservatives would “sup with a long spoon” and distance themselves from Russian donors, but electoral commission records analyzed by Britain’s Sunday Times newspaper show Russian-linked donations have continued.

Contributions have also been given to the Conservatives by British lobbyists and PR firms working for Russian oligarchs and even the Kremlin. New Century Media, a PR agency contracted by the Russian government to manage a marketing campaign in Britain to present a “positive image” of Russia, has donated more than $200,000 to Britain’s ruling party.

Opposition lawmakers called Sunday for the Conservatives to return the donations, arguing they raise doubts about the government’s determination to retaliate for what the country’s intelligence agencies believe was a Kremlin-approved attempt on March 4 to kill on British soil Sergei Skripal, a former Russian military intelligence officer who spied for the British, and his 33-year-old daughter.

British officials suspect the nerve agent used — likely either modified Sarin or VX — was developed near Moscow at the Yasenevo laboratory run by Russia’s intelligence service the FSB. Russian officials have dismissed the claims of FSB or Kremlin involvement in the assassination bid that has left father and daughter critically ill. They say the allegations are wild and hysterical, part of a Western campaign to demonize Russia.

The donations to the Conservatives “call into question how seriously Theresa May will be willing to challenge Russia’s conduct when her party is literally being bankrolled by some close allies of the Kremlin,” said Nia Griffith, the opposition Labour Party’s defense spokesperson.

In a statement, Britain’s Conservatives said, “All donations are properly and transparently declared to the Electoral Commission.”

The most generous Russian donors to the Tories include Lubov Chernukhin, wife of former Putin finance minister, Vladimir Chernukhin, who has given more than $600,000 to the Conservatives since 2010. She bid successfully at a party fundraising auction to play tennis with former Prime Minister David Cameron and Britain’s foreign secretary, Boris Johnson. Her bid was $200,000.

Conservative officials say that Chernukhin, now a British citizen, is not a “Putin crony”, arguing her husband fell out with Russian leader Vladimir Putin after being dismissed from his job running a state-owned bank.

On Monday, Britain’s National Security Council is due to meet to discuss the latest findings of the Skripal investigation and to consider what retaliatory measures to take against Russia.

Party insiders say Johnson and Defense Minister Gavin Williamson are expected to demand tough retaliation. Both privately have expressed frustration with Theresa May’s order to ministers not to rush judgment and to avoid pre-judging the investigation’s conclusions.

They will join Home Secretary Amber Rudd in calling for the introduction of a new law to target Russian officials. They want a British version of America’s so-called “Magnitsky Act,” a law passed by the U.S. Congress in 2012 that targets Russians deemed by Washington to be complicit in human rights abuses, including extra-judicial killings and torture. Specific sanctions would include visa bans and asset freezes.

Retaliation by Britain would almost certainly trigger a response by the Kremlin, say officials. When the U.S. Congress passed the Magnitsky Act, Russia banned American couples from adopting Russian children.

In an interview Saturday with VOA, Bill Browder, the American-born financier who was instrumental in persuading Congress to pass the Magnitsky Act, said he expected a major Russian-funded lobbying effort in Britain to try to persuade the British government not to retaliate. “You can bet that anyone who is making money off Russia is doing their best to keep things calm here in Britain and to stop a reaction,” he said.

Browder, who ran one of the most successful investment funds in Russia before his expulsion in 2005 when his business was expropriated, lobbied hard for U.S. sanctions to be introduced after his lawyer Sergei Magnitsky was arrested and died in Russian custody.

He worries the British will be limp in response to the attempted assassination, pointing out the cautious May has opposed past retaliation against Russia because of the risk of disruption to British business.

Browder says he has no doubt the Russian government was behind the assassination attempt on Skripal, dismissing suggestions it may have been a rogue operation. “No one would have the guts to go rogue. These operations are approved and planned by the Kremlin,” he says.

Britain does have a way of deterring the Kremlin, he argues. “Britain has gigantic leverage. All the Russians, members of Putin’s regime, come to London. They buy expensive property, they open bank accounts here, they send their kids to private schools, and so the easiest thing to do is seize their properties and ban their travel and that of their family members. That will immediately cause them never to do this again.”