The Conservative party is under pressure to reveal details about its relationship with the London arm of a US lobbying firm accused of smear tactics against critics of Facebook.

UK Policy Group, a consultancy with close links to the Conservative party, is part of Definers Public Affairs, the controversial firm ditched by Facebook earlier this month following a New York Times exposé that has further dented the social media network’s image.

The newspaper alleged that Definers encouraged reporters to investigate whether there were financial links between George Soros and an anti-Facebook movement, a tactic that led to claims of pushing an antisemitic trope by portraying the billionaire Jewish philanthropist as a wealthy puppetmaster.

Sheryl Sandberg, Facebook’s chief operating officer, said it was “never anyone’s intention to play into an antisemitic narrative against Mr Soros or anyone else”. She added: “Being Jewish is a core part of who I am and our company stands firmly against hate.” But the backlash against Facebook has been ferocious and has seen Definers’ work on both sides of the Atlantic come under forensic scrutiny.

Set up in 2017, UKPG’s vice president is Andrew Goodfellow, the Tories’ former director of policy and research. Its two founders, Matt Rhoades and Joe Pounder, are heavyweight US political operatives who – in addition to establishing Definers – set up the America Rising corporation, described as the “unofficial research arm of the Republican party”.

“Our blueprint to winning elections involves the relentless pursuit of original and effective hits against Democrats,” the firm explains on its website.

UKPG’s only known client is the Conservative party, for which it reportedly provides research on its opponents. A spokeswoman for the Conservative party confirmed it works with UKPG.

When asked whether it intended to continue working with the firm in light of recent allegations and what the firm did for the party, she said: “We don’t comment on contracts with individual suppliers.”

Jon Trickett, Labour’s shadow minister for the cabinet office, said: “It’s precisely because of revelations like this that the public have no faith in this government to clean up our democracy and restore trust in politics.”

Facebook Twitter Pinterest Definers allegedly encouraged reporters to investigate if there were financial links between George Soros and an anti-Facebook movement. Photograph: Luke MacGregor/Reuters

In addition to employing several well-connected former Tory staffers, UKPG has been able to draw on the advice of Ameet Gill, former director of strategic communications for David Cameron. Gill’s PR firm, Hanbury Strategy, which he co-founded with Paul Stephenson, director of communications for the Vote Leave campaign, was employed as an adviser to UKPG for six months after it launched.

Hanbury has also advised AggregateIQ, the Canadian data firm used by pro-Brexit groups in the referendum.

America Rising corporation is the commercial arm of America Rising political action committee (Pac), whose backers include the billionaire hedge fund manager Paul Singer. Filings show the Pac spends heavily on the services of both America Rising Corporation and Definers.

Definers set up UKPG just as concerns about Facebook’s relationship with the discredited data firm Cambridge Analytica reached fever pitch.

“Helping the private sector influence public opinion in a fast-changing political environment is an exciting challenge that UKPG will embrace head on,” Pounder explained at the time.

In the US, Definers was close to Cambridge Analytica. Its sister company, America Rising, with which it shares offices and some staff, held a joint Christmas party with the data firm in 2015, when they worked on Ted Cruz’s Republican nomination campaign.

Pounder is also editor in chief of NTK Network, an online news platform. The network, which has more than 120,000 followers on Facebook, has published many anti-Soros stories as well as those critical of Facebook rivals, notably Google and Apple – stories that have been picked up by other conservative news outlets. It has also reported favourably on Brexit and Nigel Farage.

Definers and UKPG did not respond to requests for comment.

When the Observer telephoned UKPG’s serviced offices in London’s Victoria last week, an administrator said that the firm had left the previous week. “I don’t know where they have gone,” he said.

“This isn’t any old muckraking outfit,” said Tamasin Cave of Spinwatch, a group that monitors PR agencies. “Was UK Policy Group an ‘unofficial research arm’ of the Conservative party? A pro-Brexit operation? We need to know why they arrived in the UK when they did, what they were doing for nearly 18 months, who they were targeting and who was paying for it. And why they appear to have upped sticks the very week Definers are fired by Facebook.”