David Cameron is facing calls to publish the guest list of Wednesday's Conservative summer fundraising dinner following revelations that last year's secretive event attracted bankers, businessmen and lobbyists with a combined wealth of more than £11bn.

"Cameron is trying to buy his way to power," said Jon Ashworth, the shadow Cabinet Office minister. "We will be demanding to know who is whispering in his ear at the Hurlingham Club."

The Tories have refused to publish the guest list for the annual dinner, which is the party's second biggest fundraising event. An investigation by the Guardian and the Bureau of Investigative Journalism revealed that in the week after the 2013 event, which attracted 449 guests paying up to £1,000 per seat, £1.1m in donations to the party were registered with the Electoral Commission. Attendees included 73 financiers, 47 retail and property tycoons, 10 people in oil, gas and mining and 19 working in public affairs and PR.

The 2014 event takes place on Wednesday at the Hurlingham private members club in west London. And campaigners for political transparency said both events were "a straight-up case of cash-for-access".

At Prime Minister's Questions on Wednesday, Cameron was challenged by Kerry McCarthy, the Labour MP, to reveal what was discussed between defence secretary Philip Hammond and Lord Clanwilliam, a public affairs executive employed by the government of Bahrain, who shared a table at the 2013 event.

He refused and instead attacked Labour's reliance on finance from trade unions. "The Labour party just has to get one trade union to write one cheque for £14m," he said. "Eighty percent of the candidates are union-sponsored. They have bought the candidates, they bought the policies and they bought the leader. We must never let them near the country again."

Asked to comment on the summer party, a Tory spokesman said: "All donations to the Conservative party are properly and transparently declared to the Electoral Commission, published by them, and comply fully with Electoral Commission rules."

Darren Hughes, the deputy chief executive of the Electoral Reform Society, said the revelations were "the latest demonstration of why we need a new, more transparent system for funding political parties".

"Every time we get an insight into how parties raise money, it further cements the impression that our democracy is for sale," he said. "Earlier this year we found that three-quarters of the public believe big money has too much influence on political parties, and 61% believe the party funding system is corrupt. Revelations like this are only going to push those numbers in one direction."

Alexandra Runswick, the director of Unlock Democracy, said: "The public feels our politics is for sale and that politicians listen to donors and lobbyists but not voters. The big problem is this isn't shocking any more. Voters have started to see this as part of the political process and it shouldn't be.

"With an event like this dinner they can fund much of a general election campaign in one evening. It is much harder to get thousands of smaller donations from the public. Labour is at fault, too, and also hosts fundraising events where you have to pay for a ticket, and who is sitting at the table is not declared."

Tamasin Cave, the director of the lobbying campaign group, Spinwatch, said the summer parties were "a straight-up case of cash-for-access".

"Buying a seat at a minister's table provides these bankers, foreign businessmen and lobbyists with an opportunity to discuss their concerns, whether its taxes, regulation or policy," she said.