Labour will lose the next election if it offers "more of the same", warns the head of the Fabian Society, Sunder Katwala.

"A bolder Labour vision is now a matter of strategic necessity, not just centre-left desire, says Fabian General Secretary Sunder Katwala, in a new paper The Vision Thing: Why Labour Can't Win Without a Progressive Agenda, launched at a Not the General Election night debate in Westminster.

Too narrow a focus on keeping swing voters in the super-marginals will be in vain if Labour does not also get its working-class vote out and reconnect with disillusioned liberal opinion too. There are 'heartland' and liberal voters in every marginal seat – not being the Tories won't be enough to secure them.

Brown should beware of advice to "stick to New Labour's > winning formula". The key arguments of the last three campaigns have lost their power. The 1997 message was 'kick the Tories out'. In 2001, Labour said "more time needed" and in 2005, Michael Howard showed that "the nasty party hasn't changed".

But memories of the Tory years have faded; the government must counter calls of 'time for a change' and everything David Cameron does is meant to send the message that the Tories have changed. Brown will need a positive argument to win.

Brown must go public with his Labour vision of a fairer society. As Prime Minister, Brown has appealed above politics and across party. But he can not achieve the 'progressive consensus' he seeks if he is only prepared to say things with which nobody could disagree. Katwala said: "Brown's initial positive appeal was built on leadership, competence and authenticity. But political vision is central to the authentic Gordon Brown. The message to his advisers should be 'let Gordon be Gordon' – and that must mean letting Gordon be Labour too."

The paper sets out five tests for a 'progressive manifesto' which can mobilise progressive constituencies for Labour.

Inequality challenge:

Find an additional £4 billion to get on track towards the 2010 child poverty pledge, to make child poverty a central campaign issue.

Democracy challenge:

A referendum on electoral reform, proposing the Alternative Vote for the Commons along with a PR-elected second chamber.

Civil liberties challenge:

A 'bonfire of petty restrictions' and review ID cards.

Green challenge:

Don't back down on green taxes and national road pricing.

Foreign policy challenge:

On Iran, ensure Britain commits to a diplomatic solution, and opposes a US military confrontation with Iran.