The shadow foreign secretary said Seumas Milne, the Labour leader’s chief communications adviser, should not make comments in public as his views were “not important”.
She issued the slap-down after being asked on ITV to respond to a report — denied by Labour insiders — that Mr Milne has predicted Boris Johnson will successfully pass his Brexit deal.
“What you should do is talk to elected representatives because we are the ones who are democratically accountable,” Ms Thornberry said. “Advisers advise and frankly it’s not their job to make comments.”
She added: “His advice is always, you know, listened to with great care, but what he says is not important. What politicians say is what’s important.”
A Labour source said Mr Milne had never aired the views attributed to him.
“Seumas didn’t say that. It was made up and then regurgitated on Twitter,” said the source.
The incident follows a series of clashes between shadow cabinet members and Mr Corbyn’s office over policy, in particular over Brexit and a second referendum.
Karie Murphy, his chief of staff, was moved to Labour Party headquarters to lead digital campaigning, in what was portrayed as part of a “silent coup” against the senior Left-wingers advising Mr Corbyn.
Former Guardian journalist Mr Milne was widely seen as the next target.
Lord Kerslake, a former head of the civil service, has been advising Mr Corbyn and shadow chancellor John McDonnell on preparing to move into government and reportedly broke the news to Ms Murphy that she was being moved. Ms Murphy and Mr Milne have been the two closest advisers to Mr Corbyn since he was elected leader in 2015, and they have been seen as blocking moves to shift Labour onto a more open pro-Remain stance.
In the Commons last week, Boris Johnson pointed his finger at Mr McDonnell for being behind what he called “the Soviet era expulsions that are taking place in [Mr Corbyn’s] circle, as one-by-one his lieutenants are purged as Lenin purged the associates of poor old Trotsky”.
Tensions were exacerbated by the recent resignation of Andrew Fisher, Labour’s policy chief, following clashes with colleagues in the leader’s office.