Government backing for new forms of gas extraction such as "fracking" are under acute scrutiny after a sacked energy minister warned against "betting the farm" on them and green groups expressed alarm at links between the fossil fuel lobby and the Tories.

The Observer has established that two key executives of the energy trading giant Vitol, whose boss has given more than £500,000 to the Conservatives, were personal shareholders in a company bringing "hydraulic fracturing", commonly known as fracking, and a related technology, coal bed methane (CBM) extraction, to the UK.

But doubts are growing over whether such technologies can deliver the cheap energy prices the gas lobby claims.

Writing for the Observer, former energy minister Charles Hendry, who lost his job in the reshuffle, warns that shale gas "cannot bring the UK the same benefits as in America, where consents are much easier and prices are kept artificially low by the lack of export facilities".

After a week in which David Cameron staked his credibility on reducing energy bills, Hendry writes: "We may face a golden age for gas, but don't assume it will be cheap. Last year's energy price rises owed more to rising global wholesale gas prices than anything else, so betting the farm on shale brings serious risks of future price rises."

The previously unpublicised interests of two Vitol executives, Bob Finch, its head of trading, and Christopher Bake, managing director of Vitol, Dubai, in "unconventional gas extraction" assets suggests they believe it could be a major source of energy.

Alan Duncan, the international development minister, worked for Vitol in the 1990s and was a consultant for another company part-owned by Vitol.

Ian Taylor, its chief executive, who has donated around £550,000 to the Conservatives, was a guest at an intimate dinner party with David Cameron in his Downing Street flat last November. Weeks after the private dinner - for people who had donated more than £50,000 to the Tories - it emerged that Vitol had supplied oil to rebels in Libya. The revelation prompted suggestions the deal had been brokered by the Foreign Office, a claim comprehensively rejected by Vitol.

Last month, Vitol admitted it had bought and sold Iranian fuel oil. The Swiss-based company said: "A Bahraini subsidiary company purchased a spot cargo of fuel oil from a non-Iranian counterparty in July 2012. The fuel oil delivered ... was of Iranian origin. Vitol Group companies no longer purchase any product of Iranian origin."

Joss Garman, a campaigner with Greenpeace, said: "Call me cautious, but these don't sound like the kind of people we want to be entrusting with our land, countryside and climate. The government urgently needs to kick the UK's gas habit in order to stabilise energy bills."

Lord Browne, the former chief executive of BP and now chairman of Cuadrilla, one of the UK's main shale prospectors, is an adviser to the government. He is also managing director of Riverstone Holdings, which has oil and gas investments in the North Sea. Ben Moxham, who worked for Browne at BP and Riverstone, is now Cameron's energy adviser.

A Cuadrilla site in Lancashire was forced to suspend test fracking in 2011 after two small earth tremors. In April a panel of experts appointed by the government ruled that test fracking could continue under stringent conditions.

Finch and Bake, along with two venture capital firms, acquired a 16% stake in a company called Dart Energy after selling it to a company called Greenpark that has a licence to conduct exploratory fracking at a site on the Anglo-Scottish border. Finch no longer has an interest in Dart, according to Vitol. Next February, Dart plans to start drilling 22 CBM wells on the bank of the river Forth, the largest venture of its kind in the UK.

Dart is a member of the Australian Petroleum Production and Exploration Association, which employs Australian lobbyists Crosby Textor. A former Crosby Textor lobbyist, Guy Robinson, is special adviser to the environment secretary, Owen Paterson. Paterson, accused by green groups of being a climate-change sceptic, has voiced support for fracking, describing shale reserves as "one unexpected and potentially huge windfall".

Both fracking and CBM extraction are fiercely opposed by environmental groups. Fracking involves injecting fluid under high pressure to crack rocks underground and release the natural shale gas. The technique is banned in several countries, including France.

• This story was amended on 2 November 2012 to remove incorrect references to the Vitol Group being a shareholder in Dart Energy and to include Vitol's denial that the Foreign Office brokered a deal to supply oil to Libyan rebels