A senior union chief has warned that Labour could be finished as a political force unless there is a fundamental change in the style of government.

Writing for the Guardian, Kevin Curran, the general secretary of the GMB union, says that Labour runs the risk of losing the next general election.

"If the Labour Party fails to ditch the New Labour approach in its commitment for a radical third term then I fear the consequences," he says.

"There is a real possibility of Labour losing the election and the party falling apart unless there is that change in direction."

And he warns that trade unions could desert the government unless Number 10 engages more with its grassroots.

"Should we get mid-way through a third-term Labour government and there is no change of direction in terms of policy, then in all conscience I could not ask GMB members to continue affiliating to the Labour Party," he writes.

"I want to maintain the link but the mood in the Labour Party in the country, among constituency activists and trade union members, is to reduce funding and support."

The left-wing union leader issues a rallying cry to his colleagues in the union movement.

"The responsibility facing us is huge and not to be underestimated. We are at a watershed. The interests of the Labour Party and the trade union movement will always be interlinked so long as both understand that the purpose of our endeavours is economic and social justice," he said.

"But if the unions are not able to support the party, the party will lose its conscience - becoming a sterile organisation focused on winning power with no moral or political philosophy."

The GMB voted this week to withdraw funding to the Labour election campaign, preferring to support union-friendly MPs directly as well as its own political projects.

The move prompted Blairite MPs to launch an attack on Curran.