A UK government trade summit in Saudi Arabia has been criticised for helping to promote businesses linked to a string of senior Conservatives, including peers and the former chair of the party, Ben Elliot.

Oliver Dowden, the deputy prime minister, has been in Riyadh this week launching the government-backed Great Futures campaign to promote British trade with Saudi Arabia, despite the Gulf country’s controversial record on the repression of women and LGBT people.

Dowden led the trip of 450 British delegates alongside the culture secretary, Lucy Frazer, while the business secretary, Kemi Badenoch, was in Riyadh for trade talks on Tuesday. Rishi Sunak recorded a video message of endorsement for the summit.

Among the UK delegates were three prominent Conservative peers employed by businesses with a presence at the Riyadh summit: Jo Johnson, a former education minister, who is the chair of FutureLearn, an education business, Philip Hammond, the former chancellor, who is the chair of an urban development firm called Innovo, which was sponsoring the event, and Eddie Lister, the former chief of staff to Boris Johnson, who is an unpaid director of the Saudi British Joint Business Council and employed by two of its members.

Jo Johnson, the former universities minister, now a Conservative peer, was pictured signing a memorandum of understanding with a Saudi entity at the summit on behalf of FutureLearn and spoke on the main stage as part of a panel, while Lister also spoke on the stage for an event on infrastructure.

Hammond hosted a side event on behalf of the Saudi British Joint Business Council, where he is an unpaid member of the advisory board, and was accompanied by Dominic Johnson, a trade minister and also a Conservative peer.

Campaign groups called for more transparency about the event, with Tom Brake of Unlock Democracy saying: “With a number of prominent party grandees attending the Riyadh summit, and supported in their efforts by the UK government, total transparency is needed on who was invited to attend, why and the costs.”

Rose Whiffen, at Transparency International UK, said: “Those in positions of power should always avoid the perception of handing out access and influence to their friends and supporters.”

Daisy Cooper, the Lib Dem deputy leader, said the Conservatives had “serious questions to answer” about what appeared to be a clear conflict of interest, while Labour MP Fabian Hamilton, a member of the foreign affairs select committee, said Dowden had questions to answer about whether the summit was “helping to drum up business for his Tory friends”.

Hammond said it was “preposterous” to criticise the summit for featuring Conservatives. “These missions are open to all British businesses. You surely are not suggesting that having any connection with a Conservative should make a business ineligible … The DIT’s job is to promote British trade. That is what this mission was doing.”

He said he did not fly on any government-organised flight, that his role at the Saudi British Business Council was pro bono and that Innovo was one of many sponsors of the Great Futures event.

Lister said criticism of the summit was “rubbish”, adding: “I am a director of the Saudi British Business Council and I have attended in that capacity and my role is to promote British business in Saudi Arabia. My speaking was all on construction and I have no personal interest in any construction in Saudi Arabia.”

Other prominent Conservative-linked people to attend include Ben Elliott, the former Tory chair and ex-fundraiser for the party, who runs luxury concierge service Quintessentially. He was present at a function seated on a table with Lucy Frazer, the culture secretary, and a Saudi minister for tourism.

Amanda Staveley, the co-owner of Newcastle United football club, along with the Saudi Public Investment Fund, who is also a Conservative donor and has strong connections to investors in the Middle East, was also present at the event and spoke on a panel.

Asked about the presence of businesses with Conservative links at the summit, a government spokesperson said: “Great Futures participants were all British businesses invited through a rigorous and politically impartial process based on their potential to expand through a strengthened UK-Saudi economic partnership.

“The summit was attended by a range of senior UK and Saudi business, cultural, creative industries and government figures.

“Great Futures is the largest UK trade delegation of the last decade with more than 450 UK attendees. It is helping to grow the economy and create more jobs across the UK by promoting British business.”

The government had no response to a request for the cost of the summit but it has previously argued that such summits provide a return on investment in terms of securing new investment for the UK.