Almost two dozen employees from companies including the energy giants British Gas and npower are working at the Department of Energy and, in most cases, are being paid by the government to do so, documents released under freedom of information rules reveal. Oil companies such as Shell and ConocoPhillips also have staff inside the department, and civil servants have travelled in the opposite direction to work for the companies.

The Green party MP Caroline Lucas, who made some of the FOI requests, said: "Fossil fuel giants should have no place at the heart of government given that their current investment strategies run contrary to the need to build a low-carbon future that delivers both security and prosperity. It's even more outrageous that taxpayers are footing the bill for some of these secondments, including from British Gas-owner Centrica, at a time when British Gas customers are struggling in the face of a 6% rise in their energy bills, and the company is expected to make £1.4bn profits after tax this year.

"These corporations obviously don't lend out their employees without expecting something in return."

A spokeswoman for the Department of Energy and Climate Change said: "Secondees bring with them knowledge and expertise which are vital to helping Decc do its job effectively. Likewise, seconding Decc staff into industry – be that oil, gas, renewables or other areas – provides insight into the challenges faced by those sectors. It is normal for secondees to be paid directly by Decc or for their company to be refunded for their time; this is standard practice across government."

The documents show 23 external people are working at Decc. Those being paid by the department include employees of Centrica, Barclays, ESB (Ireland's biggest energy firm), National Grid and Rolls-Royce. Staff from Shell and npower's owner, RWE, are seconded to Decc for free.

The contracts signed by secondees ask them to self-police any conflicts of interest. It states: "It is a condition of the secondment that the secondee ensures to the best of their ability that in the course of their duties for Decc there will be no conflict of interest or perception of such."

The Decc spokeswoman said: "We ensure any secondee is bound by relevant professional codes of practice. Contractual measures make sure that any secondee is not placed in a position where they could be a conflict of interest."

Joss Garman, political director of Greenpeace, said: "Centrica, Shell and RWE are corporations that make their huge profits from the fossil fuels that are warming our planet. Nobody will believe that these vested interests are lending staff to Decc out of charity. To the contrary, they have a clear financial interest in putting their people into key positions where they can exert a malign influence that runs counter to the public interest."

After revelations in 2011 in the Guardian about staff from companies working in Decc, Lucas wrote to the then cabinet secretary, Gus O'Donnell, stating: "The perception exists that [people and expertise] can be loaned out in return for access to ministers and valuable influence over the processes and decisions of government. I'm sure you will agree that such a perception is damaging." Lucas received no reply.

She said: "To avoid serious damage to public trust in government, secondments must at the very least be transparent and properly monitored, to ensure that such arrangement cannot be exploited by vested interests against the public good."

A Guardian analysis of declared meetings showed there were 195 meetings between Decc ministers and energy companies and their lobby groups between the 2010 general election and March 2011. There were 17 meetings with green campaign groups.