Gordon Brown’s allies now realise how hard it is to be Prime Minister, Alastair Campbell has said.

In a foreword to the paperback version of his diaries, Tony Blair’s former press secretary says that Mr Brown’s friends underestimated the scale of the task facing him.

But in an interview with The Times last night Mr Campbell called on Labour MPs to back Mr Brown at a difficult time. “Because we have won three elections and been in power for a considerable period of time, there are people in the Labour ranks who think winning is easy,” he said. “It has always been difficult.”

In Dublin, where he was on a speaking engagement, Mr Campbell said: “Some of the MPs who have been causing difficulty for the leadership would be better placed devoting their energies to putting pressure on the Conservative Opposition, who are getting away with murder. In the absence of the media putting the Conservatives under pressure it is incumbent on Labour MPs to do so.”

In the foreword Mr Campbell writes that one or two people around Mr Brown when he was Chancellor sometimes thought he was doing a slightly smaller version of Mr Blair’s job. And because the economy was so central to the Government’s success, he was doing it better.

“They now know just how hard it is. How relentless the workload and the pressures are. How many issues can come along and knock you. How much of your time and energy is devoted to trying to remain strategic amid pressures to deal with the day-to-day.

“How much of the media is constantly trying to trip you up, and even the ones you think supportive – with a few exceptions – are looking to flake off. How hard it is sometimes to keep your own side happy and on board, let alone those not naturally disposed towards you.”

Mr Campbell’s words were written at the turn of the year after Mr Brown had been hit by problems including Northern Rock, lost computer discs and party funding. They were written without any sense of gloating and Mr Campbell makes plain he is a strong supporter of Mr Brown’s efforts to lead Labour to a fourth term.

But they give strong credence to the view that Mr Brown has found the job much bigger than he expected, something he has admitted in private. He has told colleagues that bringing in Stephen Carter to be his strategic chief and making Jeremy Heywood his Permanent Secretary at Downing Street was an acknowledgement that he needed more than his trusted former Treasury aides around him.

Mr Campbell tries to demolish what he calls the current narrative that while Mr Brown struggles, it was all plain sailing during the Blair years. His diaries showed that once the honeymoon was over there were difficult and damaging issues. “There may be good days and bad days. There is no such thing as an easy day.”

He adds: “If there is a lesson from this book, it is that whilst politics and government are full of the difficult and the unexpected, and the occasional genuine crisis, provided you have values in touch with the times, a robust forward-looking modernising strategy, clear dividing lines where you want them vis-à-vis your opponents, the stamina and energy needed for the campaign, and the ability to deal with crises when they come, you should come out on top. So for all the difficulties, Gordon is well placed.”

Mr Cambell, who suffered exhaustion and depression after No 10, is working on a novel, All in the Mind, which has as its hero a psychiatrist.

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