The leader of Britain's largest union has warned that the Government has "lost its way".
But Unison general secretary Dave Prentis said that changing the leader is not the answer to Labour's woes, as no other potential Prime Minister would do better than Gordon Brown.
Meanwhile, another union boss has launched a fierce assault on the Cabinet minister most often tipped as a successor to Mr Brown, Foreign Secretary David Miliband.
In an interview on the eve of the TUC's annual Congress, Unite's joint general secretary Derek Simpson branded Mr Miliband "smug" and "arrogant", and said the country might as well give power to Conservative leader David Cameron.
Mr Simpson told The Observer that there is no-one in the Labour Party - with the exception of left-wing backbencher Jon Cruddas - who could do a better job than Mr Brown.
But he said it was vital that Mr Brown stop "pandering to Tories inside the Labour Party".
Meanwhile, Mr Prentis told the paper that Labour voters and union members were "fed up to the back teeth" with a leadership that never acted in their interests, but those of big business.
"In everything that has happened, our Government has lost its way," said Mr Prentis.
But he said changing leader would "make no difference", asking: "Who has got more experience than the current Prime Minister? Who could we put in who is going to come in with the panacea to cure all these problems? They are not around. The problems are too serious."
The union leaders spoke out ahead of what promises to be a bruising TUC gathering in Brighton for Mr Brown and the Government.
