Labour leadership hopeful Emily Thornberry said Jeremy Corbyn has been "weakened" by his advisers during his time as leader.
While she did not mention any of Mr Corbyn's advisers by name, Ms Thornberry said there were a "number of issues" which gave voters the impression that the Labour leader was indecisive.
Speaking on the Peston show on Wednesday, the shadow Foreign Secretary officially confirmed she was in the running for Labour's top job.
She said: "I think there were a number of issues. Obviously the one the Tories have milked for all its worth is Jeremy's reaction to Salisbury, but most importantly it was the briefing after Jeremy made his statement on Salisbury which was not on all fours with what Jeremy had said.
"And so the briefing to the press thereafter gave a different impression."
Ms Thornberry refused to discuss who was behind the briefing after Peston suggested media reports had claimed Seamus Milne - Mr Corbyn's chief adviser - was responsible for the briefing.
She added: "But the point is, what you need to have in a leadership is to make sure its the leaders who decide and its the advisers who advise and there should not be any mixing up of the two.
"Frankly you should have advisers who give advice but the politicians make the decisions, and the politicians make those decisions and do not try and go back on it.
"By doing that, and it was done too often, I think that it weakened Jeremy, it weakened the impression he gave people and it looked like he wasn't certain."
The shadow foreign secretary was also critical of the Labour leadership's decision to agree to Boris Johnson's call for an early general election without being "clear" with the country on its Brexit stance.
Ms Thornberry - who has advocated for the UK to Remain as part of the EU - said she tried to express her concerns about going into a general election with Mr Corbyn prior to the campaign.
She said: "It was difficult to get hold of Jeremy, so I put in black and white what I thought and also sent a letter to the Leader's Office. And I had a number of conversations with those in the leaders office.
"But in order to make it clear that strategically it was going to be a catastrophic political mistake to go into a leadership election on a single issue when we were not sufficiently clear about that issue when all the other parties were really keen to have it on a single issue because they were hoping to eat the Labour vote."
Matt Hancock was also on the show, the first since the general election campaign, and the NHS Secretary refused to be drawn on waiting times in the public health service.