A Downing Street aide who caught swine flu was banned from the G8 summit because of fears that he might infect world leaders.

Gordon Brown imposed the prohibition on Michael Jacobs, his senior adviser on climate change, despite the official’s insistence that he was no longer contagious and his plea that he should be at the gathering.

Mr Jacobs had worked for much of the past year, with officials from other G8 countries, trying to put together a plan that might eventually lead to a worldwide global-warming deal at the Copenhagen summit in November.

He was delighted that, as the G8 approached, it appeared that agreement was close on a ground-breaking accord that the Earth’s temperature should not be allowed to rise by more than 2C. In the event the deal was done, and the main polluting countries joining the summit later also accepted it.

Mr Jacobs was prevented from attending however because of his illness, which he is believed to have picked up several weeks ago while in Mexico, working on the climate-change deal. Mexico is one of the leading polluting countries outside the G8.

A friend of Mr Jacobs said that he had been devastated not to be at the summit. He would have played an important part in it, attending most of the bilateral head-to-head leader meetings with Mr Brown.

On realising that he had swine flu Mr Jacobs stayed away from No 10. “We did not let him anywhere near Gordon,” an official said. Initially, he worked from home but by the beginning of last week Mr Jacobs believed that he was clear of the infection.

When he told colleagues in No 10 that he intended to travel to the summit at L’Aquila in Italy as planned there was consternation in the building, sources have disclosed.

Mr Brown told Mr Jacobs that, regrettable though it was, he could not go. After much pleading the Prime Minister was apparently ready to relent and said that his adviser could attend if his GP could give a 100 per cent guarantee that he was no longer infectious. When Mr Jacobs’ doctor was told of the event to which his patient was so eager to travel, he appears to have felt unable to do so.

Mr Jacobs’ seat on the Prime Minister’s aircraft was left empty. He was not quite grounded. In the end he went to Rome under his own steam, and spent much of the summit on the telephone to British officials and Mr Brown in the finance police barracks, the venue for the summit.

A friend said: “At least he got a piece of the action but he was mortified at not being with us.”

British officials said the American delegation had passed on their gratitude that President Obama had not been placed at risk.

It is not the first time misfortune has befallen Mr Jacobs, former secretary of the Fabian Society. While he was in China last year it was alleged that he had been the victim of an operation by Chinese intelligence agents after having his Blackberry mobile telephone stolen, apparently by a woman who approached him in a Shanghai hotel discotheque. He reported the theft to the Prime Minister’s Special Branch protection team.

Mr Jacobs worked for Mr Brown while he was Chancellor, advising him on public service reform. As head of the Fabian Society, Mr Jacobs was an avowed “tax and spend” advocate, arguing that the public would stand for open tax rises if they were explicitly tied to improved results from the public sector. He was a critic of the “ideological timidity” of the Government in its second term.

He quit the society in 2004 to pursue a research fellowship in Australia.

Mr Jacobs was said yesterday to have recovered. “We think he’s OK now but there was just too much of a risk last week,” a friend said.

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