The first case of swine flu has struck Downing Street and it nearly caused a diplomatic crisis.

Gordon Brown's senior climate change adviser Michael Jacobs was banned from attending the G8 summit in Italy for fear he would pass the contagious disease to Barack Obama and other world leaders.

It is understood that Jacobs contracted the disease while involved in climate change talks in Mexico.

He had travelled to Rome for some preliminary negotiations on the draft of the G8 communique text, and was told by his personal doctor that he was no longer suffering from the disease. He then planned to travel to the conference site in L'Aquila, Italy, but was told by Brown that he could not risk him going.

The prime minister told Jacobs it would be diplomatically disastrous if Britain was responsible for infecting the G8's leaders. Instead, Jacobs followed negotiations by phone.

A Downing Street source said there was no evidence that anyone else in Brown's entourage has contracted swine flu and that if they had, proper procedures for decontamination will be followed.

Jacobs is seen as the one of the best informed climate change specialists in Britain and his absence from the talks was regarded as a significant loss. He made no mention of contracting the disease or the ban imposed on him when he sent out a circular to those interested in climate change setting out the outcome of the negotiations, and the problems that lie ahead in securing a deal at Copenhagen at the end of the year.

Jacobs, former general secretary of the Fabian Society, clearly did not regard his absence as fatal to the outcome of the summit since he pointed out in his email to green groups that five big achievements had been secured at the L'Aquila talks,

For the first time the G8 and developing nations agreed that the science demanded global average temperatures rise by only 2C on preindustrial levels.

"Until a few weeks ago, in fact in the case of the developing countries until a few days ago we did not believe we were going to get this agreement," he said.

Secondly, the G8 agreed to cut its own emissions by 80% by 2050.

He also said it was now possible to see an agreement to cut global emissions by half at Copenhagen, the aim of the talks. The G8 meetings had seen developing countries for the first time accept the concept that their emissions were peaking, Jacobs said.