Unite chief Len McCluskey's warning for Corbyn
Jeremy Corbyn can no longer "say the first thing that comes into his head" now he is Labour Party leader, Unite boss Len McCluskey has said.
The union general secretary said Mr Corbyn had to "come to terms" with his leadership, the York Press reported.
Unite, Britain's largest trade union and Labour's biggest financial backer, supported Mr Corbyn in last summer's Labour leadership contest.
Mr McCluskey later said Mr Corbyn had his "full support".
Unite said he had expressed similar sentiments in his speech at York University.
According to the York Press, Mr McCluskey said: "He [Mr Corbyn] has been a very principled MP and been able to say what he likes, but now he's a leader and in leadership he can't necessarily say the first thing that comes into his head. He has to take some balance."
He also said Mr Corbyn's comments on shoot-to-kill powers for police were "inappropriate".
Last week Mr Corbyn told the BBC he was unhappy with a shoot-to-kill policy "in general", later saying he backed any "strictly necessary force" needed to protect the UK in a terrorist attack.
In a subsequent "clarification" of his comments, Mr McCluskey said Mr Corbyn had "opened up debate and democracy across the Labour Party", adding: "It is exactly his brand of conviction politics and principled opposition that has won him so many supporters and his leadership is stronger for it."
Mr Corbyn is attempting to find agreement within his shadow cabinet over whether to back UK air strikes over Syria. The Labour leader has indicated his opposition to military intervention but some of his front bench are in favour.
Prime Minister David Cameron is to make his case for air strikes to MPs on Thursday ahead of an imminent Commons vote.
BBC assistant political editor Norman Smith said Mr Corbyn had urged his shadow ministers to consult their constituency parties over the weekend as he seeks a "collective" view.
Labour is expected to attempt to finalise its position on Syria when its MPs meet on Monday night.