Gordon Brown has been embroiled in a spin row after "opening" a hospital unit that admitted its first patients more than three months ago.

The Prime Minister paid a high-profile visit to Basildon University Hospital, Essex, yesterday to highlight government plans to reform the NHS.

Downing Street said he had "officially opened" the Essex Cardiothoracic Centre at the hospital, which is in a marginal Labour constituency. In fact the first patients arrived at the specialist heart and lung unit on 1 July and Alan Johnson, the Health Secretary, visited the centre in the same month.

The Labour MP Angela Smith, who held Basildon in the general election two years ago by just 3,142 votes, was at Mr Brown's side as he toured the £60m centre.

As he examined its facilities and met surgeons and nurses, he remained coy on the likelihood of a snap election. Asked to smile if the electorate would be going to the polls next month, he turned away grinning.

Despite his reticence, the visit looked like a valuable piece of early electioneering for a prime minister still contemplating going to the country next month.

Peter Cripps, 67, who held the ribbon for Mr Brown to cut, was invited to return to the hospital for the ceremony, almost three months after he underwent heart surgery there. He said: "They just said there was a reception and could I come down, I didn't know the Prime Minister would be here."

The Prime Minister's visit came 96 days after the local hospital trust published a photograph of the keys to the state-of-the-art unit being handed over. On 2 July, Basildon and Thurrock University Hospitals NHS Trust issued a press release announcing that the first operation had been carried out at the unit. At the end of the month, Mr Johnson toured the centre which he said already boasted world-class standards "only four weeks after opening".

In August, the trust announced that the centre's education suite had been "officially opened" by Bernard Ribeiro, president of the Royal College of Surgeons.

The Health Secretary was back in the unit yesterday at the Prime Minister's side for the visit which took staff by surprise.

They were accompanied by Lord Darzi, the Health minister, whose report on the future of the NHS was published yesterday, a week earlier than expected, renewing speculation that an election could be imminent.

A Downing Street spokeswoman said it was usual practice for the official opening ceremony of a public building to take place well after it had become operational. She said Mr Johnson had looked round the unit in July as part of a fact-finding visit to see its facilities. She added: "You would expect him to visit centres of excellence." Basildon University Hospital confirmed it had written to Mr Brown in June to invite him officially to open the unit and proposed a date in October.

The Conservatives said the "so-called opening" had been a sham which followed a succession of fake announcements by the Government.

George Osborne, the shadow Chancellor, said: "After 100 days of spin, Gordon Brown has set a new low by faking a hospital opening. Gordon Brown's big health launch is rapidly falling apart. How can you trust a prime minister who even fiddles a photo shoot?"

The controversy comes days after James Purnell, the Culture Secretary, faced embarrassment after it emerged a hospital publicity photograph had been digitally altered to include his image. A fortnight before he had warned broadcasters of the importance of retaining the trust of viewers.

The Prime Minister was also accused this week of turning British forces into a political football by travelling to Iraq during the Conservative Party conference to announce that 1,000 troops would be home by Christmas. It emerged later that 500 of these had already been announced, 270 of whom were now back in Britain.

In the run-up to succeeding Tony Blair, Mr Brown repeatedly said he wanted to put the days of spin behind him. His critics say that he is addicted to media manipulation and has a track record of re-announcing policy initiatives several times.