Jeremy Corbyn has led Labour to its lowest level of support in polling history, as the party slumps to fourth place.
Just 18% of voters are now backing Labour in the YouGov poll for The Times - down two points from a week ago.
Despite the chaos of Brexit and a new leader still not in place, the Tories have managed to climb two points to stand at 24% while the Brexit Party is up one point to 23% - on the week they turned their backs on the EU anthem - and the Liberal Democrats are also up one to 20%.
YouGov/Times:— Henry Zeffman (@hzeffman) July 3, 2019
Con: 24% (+2)
Brexit: 23% (+1)
Lib Dem: 20% (+1)
Labour: 18% (-2)
Green: 9% (-1)
Labour have only ever been on 18% once since polling began in the 1940s. That was in an Ipsos Mori poll in May 2009
With Mr Corbyn's Brexit strategy under intense scrutiny, the survey found that only 25% of Remain backers intend to support Labour.
This compares with 48% of Remainers who said they would vote for the party at the beginning of the year, and 40% who gave it their backing at the end of April.
Just 8% of Leave supporters say they will vote Labour, compared with 21% in January, according to the poll.
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Labour's shadow health secretary Jon Ashworth told ITV's Peston: "I don't believe that would be the result at a general election.
"If that was a result at a general election it would be devastating for the Labour Party."
The polling comes as Labour was accused of not doing enough to combat anti-Semitism in the party following the readmission of MP Chris Williamson to the party last week.
It also follows a meeting where the head of the civil service said that senior officials reported to have made comments about Mr Corbyn's health did so to damage the Labour leader's reputation.
Sir Mark Sedwill expressed the view that he was appalled to read the comments that Mr Corbyn was not "physically or mentally" up to the job of Prime Minister during a meeting with the Labour leader to discuss the matter, it is understood.
A Cabinet Office probe into the controversy launched by the civil service will have an independent element, Labour said after a 45-minute discussion of the matter between Sir Mark and Mr Corbyn.
It is understood that at the meeting Sir Mark made it clear that civil servants conducting the inquiry are not connected with the event where the comments are said to have been made.
The meeting came in the wake of demands by Mr Corbyn for a fully independent probe into the matter.
A Labour Party spokesperson said: "The meeting was frank and detailed, with a full exchange of views.
"The seriousness of the civil service breach and the evident malicious intent behind it was acknowledged by all participants in the meeting.”
The row was triggered by a report in The Times which said the future of Mr Corbyn, 70, was openly discussed at an event attended by mandarins amid suggestions he has become "too frail and is losing his memory”.
One civil servant was quoted as saying "there must be senior people in the party who know that he is not functioning on all cylinders", while another said "there is a real worry that the Labour leader isn't up to the job physically or mentally but is being propped up by those around him”.