Jeremy Corbyn has been warned he can no longer "say the first thing that comes into his head" amid tensions on the Labour front bench about whether to support British air strikes in Syria.
Unite boss Len McCluskey - who endorsed the veteran left-winger's bid for the leadership - said he was still coming to terms with the responsibilities of his role, and needed to show "some balance".
The comments, reported by the York Press, came as Mr Corbyn met his shadow cabinet to discuss the approach to the Government's proposals for extending military action, due to be outlined by David Cameron on Thursday.
Mr Corbyn is understood to have told colleagues that he wants to establish a collective position on the issue, which is expected to be put to a vote in the Commons next week.
However, that could prove difficult as while the leader has indicated he opposes military action against Islamic State (IS), a number of shadow ministers are likely to support it or choose to abstain.
In an apparent effort to facilitate lobbying by his grassroots supporters, including the controversial Momentum group, Mr Corbyn has asked shadow cabinet members to consult their constituency parties over the weekend.
He will gather his team again after the Prime Minister's statement, and on Monday before a potentially explosive meeting of the Parliamentary Labour Party. Many MPs have been urging Mr Corbyn to avoid a damaging showdown by calling a free vote.
In an event at York University students' union, Mr McCluskey reportedly said it had been "inappropriate" for Mr Corbyn to raise doubts last week about whether police should be allowed to "shoot to kill" terrorists on the streets of Britain.
"Jeremy Corbyn has to come to terms with it (his leadership)," Mr McCluskey said. "He 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."
Acknowledging the scale of the task facing the Labour leader, Mr McCluskey added: "The only way for Jeremy Corbyn to become prime minister is if he puts forward a credible economic alternative that the British people can sign up to."
The internal challenges the party faces were underlined on Tuesday night when 14 Labour MPs rebelled to vote for Trident to be renewed in a Commons debate triggered by the SNP.
The leadership had ordered them to abstain amid efforts to resolve Mr Corbyn's support for unilateral disarmament with strong backing for the nuclear deterrent on the Labour benches.
But despite the problems a poll has shown Mr Corbyn still enjoys sky-high popularity ratings among Labour activists.
The Islington North MP was elected leader in September by 59% of members, registered supporters and affiliates.
But research by YouGov for The Times suggests 66% now think he is doing "well".
The firm canvassed opinions from a sample of those who were entitled to vote in the contest. Some 86% of people who cast a ballot for him were said to be happy with his performance.
Other polls have suggested that Labour is trailing the Tories in voting intention by around 15 percentage points.
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