Gordon Brown was not happy when he strode into the empty conference centre in Brighton to conduct early morning television interviews.

He had woken up with a hangover and it was not because he had overindulged the night before. Labour had been expecting The Sun to back the Conservatives at the next election. What no one had expected was that the newspaper would announce its decision on the night of his big speech.

The move took the shine off the Prime Minister’s day and infuriated his allies. Fewer than eight hours after launching his fightback, Mr Brown was on the back foot again. Lord Mandelson was alleged to have sworn at a News International executive when told, although he and his aides insisted that the offending word was “chumps”. When Mr Brown sat down with Adam Boulton, of Sky News, he wanted to get back to the agenda that was outlined in his 59-minute conference speech. His interrogator, and subsequent interviewers, wanted to talk about The Sun’s change of loyalty.

Doggedly, Mr Brown stuck to a prepared line that it was the people who decided elections, not newspapers. Then came questions about his leadership. At one point the Prime Minister told Mr Boulton: “You are sounding a bit like a political propagandist yourself.” He accused his interviewer of seeming to obsess about the leadership and snapped: “You have not given me the chance to talk about the economy.”

Mr Brown looked angry by the end but his ordeal was not over. He tried to walk away with a microphone still attached to his lapel, not realising that he was supposed to remain in the same seat for the next interview.

Later, in a BBC interview, Mr Brown again shrugged off The Sun’s defection, but his irritation was clear. At the end he stood up, with his interviewer still talking to the camera and, in his haste, stepped between her and the camera.

His day started to improve when he went to a school in Hove with his close ally Ed Balls, the Schools Secretary, and later received a warm welcome from the staff in the Brighton conference centre when he thanked them for their work behind the scenes.

By then his strategists and colleagues had begun exploring ways of turning The Sun’s switch to their advantage, arguing that it enhanced the image of Labour as underdogs, that Mr Brown and Lord Mandelson have tried to portray. Mr Brown went into the conference and watched as Harriet Harman roused delegates, declaring that Labour “won’t be bullied”.

The party’s deputy leader said that her party was angry at the paper’s decision but urged members to use the move to inspire them to victory. “We may be the underdog but we won’t be bullied ... this underdog is biting back,” she told the conference. She added: “I say to you: don’t get bitter, get better. Don’t get outraged, get out there. Don’t get mad, get mobilised.”

Lord Mandelson said: “I don’t think the readers want the sun to set on new Labour. The last thing Sun readers want is to see their newspaper turned into a Tory fanzine.”

David Cameron welcomed the paper’s support but insisted that it would not lead to him becoming complacent. “I think they have seen the Government is exhausted and out of ideas and they see a regenerated, refreshed Conservative Party ready to serve,” he told LBC Radio.

Alastair Campbell, the former Labour communications chief, played down the significance of the move. “If Labour lose, it will not be The Sun wot lost it,” he wrote on his blog. “The switch was entirely predictable, and had been evident for some time. It was merely a matter of when.”

He, too, caught the mood of defiance beginning to develop. “For Labour, it should actually help the feeling of fighting back that has finally been around this week. If you are the underdog, might as well have it clear that you’re the underdog.”

Mr Brown went from the conference hall to meet Labour’s new generation of candidates, who appeared upbeat about his speech, telling him that he had given them a platform to fight on. After lunch it was back to the conference to watch Mr Balls speak, then out to talk to the Sue Ryder care charity about proposals for free care at home for people with serious conditions. Then it was more interviews, including one organised by the Media Trust with young would-be reporters. Mr Brown enjoyed the chat and was almost back to the same jovial mood after his speech.

Back in the conference, Tony Woodley, the joint leader of the Unite union, ritually tore up a copy of the newspaper, provoking loud cheers.

Better news was to come for the Prime Minister. A YouGov poll for Sky News showed that the Conservative lead over Labour had halved in a week, offering evidence that his speech had made an impact. At the end of the Liberal Democrat conference Labour were on 24 per cent and the Tories on 40 per cent. The latest poll put Labour on 30 per cent and the Conservatives on 37 per cent.

Given that Nick Clegg’s party received a poll boost from its conference, Labour’s strategists know that it is too soon to get excited. Parties are usually helped by the increased public visibility that conferences give. But Mr Brown and his close circle were a lot happier than they had been earlier.

Times Recommends

• Oh no, Mr Darling: you have failed
• BBC survives dogfight over Kennel Club expose
• Where the Rosetta belongs can’t be set in stone