A handful of consultancy firms and a health insurance giant bidding for NHS contracts have been operating a discreet forum at which they receive regular briefings from senior health service managers charged with ushering in the new era of competition among its providers. The revelation has raised fears that the NHS is falling victim to a land grab by a few powerful business interests.

Documents obtained under the Freedom of Information Act by the campaign group Spinwatch shine a light on the workings of an obscure group whose existence and limited membership has alarmed campaigners who want the NHS to remain public.

The Commissioning Support Industry Group (CSIG) is largely unknown to outsiders. Its members are jockeying to win an estimated £1bn of contracts advising the new doctor-led clinical commissioning groups that will be responsible for spending more than two-thirds of the NHS budget on purchasing patient care.

Supporters say it will empower doctors and bring in competition among NHS providers. Those awarded contracts to advise doctors' groups will be involved in patient care reforms, drug purchasing, negotiating hospital contracts and, crucially, outsourcing services to the private sector.

Critics who warn the reforms will see big consultancies given contracts to advise doctors' groups say their fears are confirmed after learning more about the CSIG. A series of emails between members of the group and NHS England officials reveal that UnitedHealth, the giant US health insurer that formerly employed NHS England's chief executive, Simon Stevens, chairs the group, provides its secretariat and recently paid for senior health managers to visit its care centres in the US on a five-day fact-finding mission.

Dr Chris Exeter, UH's lobbyist, who in 2011 worked on non-health matters for Low Associates, a lobbying firm run by Sally Low, wife of former health secretary Andrew Lansley, helps co-ordinate meetings of the CSIG, whose other members are consultancies KPMG, Capita, McKinsey, EY and PWC. Its meetings, which began in May 2013, are unminuted.

"David Cameron promised us transparency, but this exposes the cat's cradle of secret connections between politicians with a financial interest in privatising the service – the big five management consultants, multinational private companies and officials running the health service," said Dr Jacky Davis, founder of Keep the NHS Public.

An agenda for June 2014 reveals that the group's members held discussions with senior NHS officials on issues including "access to specialist and niche providers" and "market rules". In response to a series of questions from Spinwatch, NHS England revealed the identity of the CSIG's members and said it "routinely" held bi-monthly meetings with Bob Ricketts, director of commissioning support services and market development at NHS England, as well as with other senior staff from the organisation.

"This exposes the extent to which the government lies about their reforms," Davis said. "GPs are not going to be at the heart of decision-making in the NHS; multinational management consultants and private companies will be advising the NHS about the big purchasing decisions and they will be advising the NHS on how to buy healthcare from the same private companies. It's like putting Dracula in charge of the blood bank."

The CSIG's role is likely to come under greater focus after NHS England recently announced the shortlist of companies and organisations competing to be on the "approved" list of firms that can bid for contracts advising GPs. The commercial providers in the running for the sought-after "lead provider" contract are Capita and a consortium led by UnitedHealth that includes its lobbying agency Hanover Media, KPMG and BT.

Emails exchanged between Exeter and NHS England managers reveal that UnitedHealth paid for an annual trip to the US for "senior-level executives from across the UK health system" to find out how the company operates in the US and to "explore their applicability in the UK". According to the emails, UnitedHealth sees the market for commissioning services in the UK "expanding and deepening" and wants to "kickstart a dialogue" on how it might emerge.

Those involved in the CSIG confirmed that it held regular meetings, but denied they were bi-monthly. A spokesperson for NHS England said: "We want GPs to be able to choose the best possible back office support to help them deliver for patients. To help achieve this, we meet all those providing such services in the public, private and voluntary sectors. There is a completely level playing field in terms of access to information. GP groups only award contracts to organisations that demonstrate the best quality and value for money for the taxpayer through an open and fair process."

A spokeswoman for Optum, UnitedHealth's British arm, said approval for the US trip was given by NHS officials and denied suggestions that the CSIG operated only in the interest of a handful of members. She said: "Several healthcare organisations engage together with NHS England three or four times a year on an ad hoc basis, in an open forum, to facilitate ease of general communications about commissioning and other NHS matters."