Ed Miliband faces renewed questions about Labour's strategic direction as a coalition of thinktanks and party intellectuals urges him not to play safe by fighting the election with a manifesto aimed only at exploiting Conservative unpopularity.

In a letter published in the Guardian, 19 leading figures from groups including the Fabian Society, Compass, Policy Network and Progress express unease that Miliband may not secure a mandate for the kind of change they believe Britain needs.

It has been signed by thinktanks from the party's left and right as well as some broader progressives, and was drafted before the publication of weekend polls showing the Tories making ground following George Osborne's budget.

Polls from YouGov and Survation showed the Conservatives neck and neck with Labour, with Labour's lead narrowing to one point.

The letter's authors argue that Britain needs "transformative" change and say: "If Labour plays the next election safe and hopes to win on the basis of Tory unpopularity, it will not have earned a mandate for such change."

The intervention suggests Miliband will face significant pressure over the coming months to send out a distinctive, clearly defined political message as the party makes its final policy decisions.

Jon Cruddas, the Labour policy review co-ordinator, has been one of those pressing for the party to reject a safety-first approach. Several policy reports are due in the next four months which, he hopes, can be welded together into a strong story of how the country needs to change the way it is governed.

The letter to the Guardian reflects a behind-the-scenes debate that has been going on in the party for months about whether Labour can win by focusing on the drop in living standards and attacking the coalition's broader record, in the hope of giving the Conservatives relatively few targets.

The letter is particularly striking given that some of signatories are on the party's centre right, such as Progress and Policy Network, and others on the left, such as key figures at Compass and Class. Other signatories include Maurice Glasman, a former ally of Miliband who has turned into one of his harshest critics, and policy experts from the New Economics Foundation, the Fabian Society, Shifting Grounds, Labour List, the London School of Economics, OpenDemocracy, the ACT! Alliance, We Own It, Changing London, the Green New Deal Group and the Global Sustainability Institute.

The thinktanks argue that the country needs a bolder approach. They suggest Britain needs a Labour or Labour-led government to meet unprecedented challenges: "A financial system still too big to fail, austerity causing unnecessary hardship to those already at the bottom of massively unequal society, climate change flooding people's homes, and a democratic system that seems pretty irrelevant to any of these problems."

But it warns that the party "must take into the election a vision of a much more equal and sustainable society and the support of a wider movement if these formidable challenges are to be met".

Chuka Umunna, the shadow business secretary, said on Sunday the front bench would not get "obsessed" with polling. "Polls go up and down. We've seen them fluctuate and, frankly, if we obsessed with polls, we wouldn't be doing our job properly," he told the Andrew Marr Show.

Labour's falling lead in the polls appears to have shaken some of the party's backbenchers, with some feeling that Miliband struggled with his response to Osborne's surprise budget announcement on pensions in the Commons last week. The only Labour MP to have gone public with complaints in recent days is John Mann, MP for Bassetlaw, saying the Conservative catch-up is a "warning shot and it would be naive to think otherwise".

"I think the message is that we need to be much clearer and I think simpler in putting across what our alternative is and what we stand for," he said on Sunday, directing his remarks at the whole shadow cabinet and not just Miliband.

The party is also facing disquiet over how far it should back the chancellor's pension reforms allowing people to take out large lump sums, which Labour this weekend said it would support in principle as long as the changes were fair, cost effective and offered good advice.

Prominent backbenchers including Tom Watson, Miliband's former election organiser, and Dame Anne Begg, chairman of the Commons pensions watchdog, as well as the shadow chief secretary Chris Leslie, have warned of some of the dangers in the coalition's proposals that may not have been properly thought-through.

The strong focus of Monday's letter, largely organised by Compass, is the disruption of the existing power networks in society by handing power to more local institutions and public service users.

They say: "National government has a continuing strategic role to play but the days of politicians doing things 'to people' are over. The era of building the capacity and platforms for people to 'do things for themselves, together' is now upon us."

The thinktanks instead suggest state institutions have to "give away power and resources to our nations regions, cities, localities and where possible directly to people". They also suggest the focus of public spending needs to shift decisively to prevention in areas such as health and the environment.

Some of these themes emerged in Miliband's recent Hugo Young lecture, but there is an appetite to make a power shift central to how Labour will deliver public services in another four years of austerity.

Some of the big policy decisions ahead for Labour include rail nationalisation, the extent to which councils will be able to borrow for new housing, the new forms of accountability for education, the financial system, and public spending decisions at a time of austerity.

In one new policy announcement due on Monday, Vernon Coaker, shadow defence secretary, will call for the next strategic review of defence to take more account of the growing threat of cyber-warfare and consider requiring private companies to report serious web attacks threatening national infrastructure.