Jeremy Corbyn has been the largest single factor behind drooping Labour support, according to a Yougov study of 200 former Labour backers.
Of 1,000 Labour supporters previously interviewed by the pollster, Yougov returned to roughly 200 who said they would no longer back the party and get the main reasons behind their rejection of Labour.
Respondents were asked to provide one or two sentences on their thinking as to why their support moved elsewhere.
And in total, 29 per cent of the 200 cited Corbyn as the main reason, while almost one in five said that the party had become a “mess” or a “shambles”.
Just over one in ten said they had no confidence in the party.
Recent polls have also shown that people who voted Labour in 2015 are almost as likely to back Theresa May as Jeremy Corbyn when asked who would make a better Prime Minister.
However, only 14 per cent of the Labour backers interviewed by Yougov even mentioned May in their responses.
Nonetheless, Yougov politics research executive Chris Curtis said there are caveats to the theory that Corbyn is the cause of Labour's problems, noting that voters may back the Labour leader to a greater extent if MPs were also supportive.
Secondly, he added that almost half of the 200 interviewed by Yougov said that they do not know which way they will vote next time.
“In previous election cycles we have found that many 'don’t knows' move back to their previous party as an election nears,” Curtis said.
“We now have a divided opposition at civil war over a leadership election and a Prime Minister enjoying her political honeymoon. If he is re-elected, Jeremy Corbyn will have the time to try and win these voters back, but he could also lose many more.”