Barker, the Tory MP for Bexhill and Battle, was offered lukewarm support by David Cameron on Wednesday following Guardian revelations over his links to an outside adviser and her private clients.

Documents released under the Freedom of Information Act (pdf) have shown that Maes introduced Barker to her clients from Air Products, a multinational energy supplier. She also asked him to intervene when her publicly funded contract was not renewed.

The Labour MP Andrew Gwynne raised the issue at prime minister's questions when he asked if ministers had learned nothing from the case of Liam Fox, the defence secretary forced to resign last year after an inquiry into his links with an adviser, Adam Werrity.

Gwynne asked: "Does the PM have the same complete confidence in his climate change minister as he did his former defence secretary?"

Cameron replied that Barker, who visited the Arctic Circle with him in 2006 to investigate climate change, was "doing an excellent job".

"I have consulted the cabinet secretary [Jeremy Heywood] over this issue, the permanent secretary of the department for climate change has been consulted on this issue and I don't see the need for a further inquiry on that basis," he told MPs.

He said the "key point" was that "the individual in question was hired by civil servants after a properly run competition".

Earlier, the prime minister's spokesman said that Cameron had confidence in Barker.

"Clearly the department has published lots of documents setting out the position. This adviser was appointed by the department, following a process run by officials and her clients were known to the department, so it's something both the Decc [Department for Energy and Climate Change] permanent secretary and the cabinet secretary have reviewed.

"The point here is that this person was appointed by the department," the spokesman said.

Asked about Heywood's involvement, the spokesman said he had got involved following earlier revelations by the Guardian and questions on the floor of the House of Commons last year. "They looked at what had happened and in this case what's very clear is that this person was appointed by the department," he said.

However, the Cabinet Office declined to answer any questions about what had prompted Heywood to launch an inquiry, when the inquiry had taken place and who had been interviewed. The Cabinet Office spokesman also refused to say whether Heywood had examined whether it was right and proper for Maes to introduce her clients to a minister while working for his ministry.

Instead, the spokesman said that the inquiry had taken place at some point over the last 10 months since a freedom of information request on their relationship was received by the department. The spokesman said: "Whilst officials were preparing the freedom of information request, the Decc permanent secretary and the Cabinet Office looked at the case."