This article is the subject of a legal complaint made on behalf of Entrepreneur Boot Camp Limited (trading as the Chemistry Club).

Companies have been paying up to £1,800 a head to meet ministers, senior government advisers and MPs at a series of networking events previously banned by the Cabinet Office.

The chief secretary to the Treasury, Danny Alexander, policing minister, Nick Herbert, and climate change minister, Lord Taylor, have all addressed the exclusive invite-only events, organised by a networking business called the Chemistry Club, and usually hosted at the high-end Sartoria restaurant in Mayfair, London.

Senior MPs from backbench committees have also attended the events, as have senior civil servants and special advisers from the Treasury, Home Office, Ministry of Defence, Department of Energy and Climate Change and other key departments.

Among the public sector employees to have attended the networking evenings is Ben Moxham, David Cameron's special adviser for energy and the environment and a former employee of BP, who was at an event on climate change in November.

The club charges senior executives from energy companies, consultancies and technology businesses between £1,300 and £1,800 per person for each event, although it invites some from the public sector to attend for free.

Senior executives from companies including BP, Shell, and the Russian oil giant Gazprom have attended the company's climate change events, while Apple, Google and Citigroup executives were among those at other networking evenings.

Transparency campaigners said the club's nature raised concerns similar to those raised in previous "cash-for-access" controversies – that those with the financial means could secure privileged access to government decision makers.

The Cabinet Office issued guidance in August 2010 to departments telling civil servants not to attend Chemistry Club events, and at the time said it had not been aware how much companies paid to attend the events,.

However, the Guardian has learned that public figures from departments and other public bodies, including the Metropolitan police and the intelligence agency GCHQ, resumed attending events after the ban, some as soon as a month after the initial guidance was issued.

The Cabinet Office now says it overturned its ruling forbidding civil servants attending after "discussions" with the club, but has not disclosed when this decision took place.

Civil servants, meanwhile, have declared their attendance at recent events on hospitality registers, frequently valuing the hospitality received at £25 or below despite the four-figure price tag for private sector attendees.

The Chemistry Club runs two main series of networking events: one, called the Climate Change Forum, focuses on energy and climate change, the other on IT and technology. The club stresses its evenings "are not social gatherings but 'work events'" that "represent an exceptionally good use of attendees' time".

A 2010 job listing for the organisation showed the emphasis the club places on making the right introductions: "The evenings are unique; prior to each private dinner every member and guest is prepared by way of a phone meeting," it said. "We gain an in-depth understanding [of] the executives' needs, resulting in relevant introductions facilitated on the night."

Mark Simon, chief executive of the Chemistry Club, said the events targeted key decision makers involved in the subject matter for invitation.

"If you look at climate change, for example, we look at who is core to the debate," he said. "There's no point inviting someone who no one wants to speak to ... The objective of the event is that they walk away having had a handful of relevant conversations.

"It's about mutuality. The civil servant wants to meet suppliers just as much as the supplier wants to meet with, say, someone at the ministry."

Simon said he could not recall if civil servants had been barred from attending the events, but his "sense is there was and then they changed it".

He believed this reversal had occurred towards the end of 2010 after discussions with the Cabinet Office on improving the transparency of the organisation, which has from last July been publishing attendees at the events on the club's website.

"If civil servants want to come, there needs to be a level of transparency," Simon said. "That's an ongoing, evolving process."

Details of attendees are now published as PDFs at the bottom of the FAQ section on the Chemistry Club's corporate site.

Simon said he had gone to see the Cabinet Office to discuss these changes. However, the Cabinet Office response to a freedom of information request from the Guardian asking for details of any meetings said it turned up no records.

The Labour MP Lisa Nandy, who has pursued further details of the Chemistry Club's activities since the Cabinet Office first discouraged officials to attend, said transparency needed to go further.

"It's been exceptionally difficult to get clear answers about who attends these meetings, and why. It's hard to avoid the impression that ministers are just paying lip service to the principle of open government. It cannot be right that lobbyists can provide access for cash," she said. "Ministers have serious questions to answer about whether lobbyists can buy influence with their government. The question remains, why would you pay thousands of pounds to network with civil servants if not to change policy or win a contract?" Tamasin Cave, of the transparency group Spinwatch, said the Chemistry Club characterised "a lobbying industry out of control".

"Lobbying is a tactical investment which affects companies' bottom line – they do not spend £1,800 for nothing," she said. Cave said the coalition's plans to increase lobbying transparency would do nothing to shed more light on events such as these.

"The government's proposals on lobbying are a joke," she added. "They do not allow us to see the impact that lobbyists have on the decisions this government makes. What we urgently need is a proper register showing who is lobbying whom, what they are lobbying for, and crucially how much money they are spending on influencing decision makers"

A Cabinet Office spokesman said: "The Chemistry Club has now changed the way it operates to provide for greater transparency around its operations and attendance enabling civil servants to attend events should they wish to do so. Civil servants' attendance at such events must be declared in departments, and will be published for the most senior civil servants."