Dave Prentis, general secretary of Unison, made the remarks as the union’s annual conference opened in Glasgow today.
The union leader said his members want to remain in the Labour Party, but warned that there are “no sacred cows”.
Mr Prentis said: “We are an integral part (of the Labour Party) and we have to build a workers’ party from within.
“We have no intention of walking away, but we will make our voice heard.”
However, Mr Prentis said there was a “fear” within Labour “they had to be very close to the Tories in what they were doing” as he expressed concerns about a possible shift to the right.
He said that Labour needs to “stress the difference with the Tories” as the party prepares to elect a leader to succeed Ed Miliband.