Ed Miliband has to do more to demonstrate he is a leader and needs to use next week's Labour conference to set out more detailed thinking behind last year's conference speech, the former cabinet minister Alan Johnson warns in the Guardian.

His remarks came as Harriet Harman declared in an interview in the Spectator that she intended to be deputy prime minister.

Labour is enjoying a 10-point lead going into its conference, but there have been a string of polls showing Miliband trailing David Cameron as the preferred prime minister.

Johnson was Miliband's first choice as shadow chancellor, but he quit after it emerged his wife was having an affair. He writes that Miliband "knows better than anyone that an opinion poll lead is not enough".

He adds: "Whilst I personally don't believe that a prime minister who is more popular than his party can deliver an election victory, it does suggest that Ed Miliband has to do more to demonstrate that he is a leader.

He says this is in part due to the "age-old problem of opposition parties who struggle to make the news and a leader who has never been tested in the difficult job he aspires to".

Johnson says he understands why Labour at this point in the electoral cycle has not yet used any policy ammunition, but hints at frustration by saying the party should at least be getting some of this policy ready, adding: "Ed needs to flesh out the thinking behind last year's conference speech."

Harman in her Spectator interview seemed to indicate Labour would not match coalition plans in the spending review next year, but she insisted she was only referring to the spending plans laid out by the coalition in 2010 – which Labour has rejected as too fast and too deep.

She was quoted as saying: "Our argument against the Tories is that the scale of and pace of their deficit reduction is self-defeating and hurting the economy and therefore making less money available. So we have got a fundamental economic critique, we would not be signing up to doing the very thing we think is hurting the economy."

Labour faces a major decision on whether to accept the spending plans, and the pace of deficit reduction to be laid out by the coalition next summer for 2015-16 onwards.

The pressure on Labour to reveal more policies and to say how it will address the deficit are underlined in an article written by two of Gordon Brown's senior former advisers at No 10, Nick Pearce and Gavin Kelly.

They say "the outlook for public spending is unremittingly bleak between now and the end of the next parliament", and suggest there will have to be hospital closures, cuts to welfare and further tax rises.

They write: "Cuts to public services and welfare entitlements will continue until at least 2017, and quite possibly the end of the decade, at which point the demographic pressure for more health and social care spending will likely crowd out the demands of many other, less politically salient services. This challenge will dominate all others and cast a shadow across every political issue of the day."

They warn: "What currently passes for radical thinking on tough tax and spending choices in centre-left Westminster circles falls far short of grasping the scale of the challenge."

• The headline of this article was amended on 27 September 2012. The original did not accurately reflect the content of the story.