George Galloway has said Jeremy Corbyn's opponents within the Labour party are preventing him from sticking to his principles and "sabotaged" his relaunch attempt last week.
Corbyn attracted severe criticism for last week's attempted reboot, particularly his demand for wage caps on high earners and subsequent volte-face.
The Labour leader has also been criticised for his perceived inconsistency on Europe and the Brexit negotiations.
Speaking on his talkRADIO, Galloway said "Jeremy has a real problem. His entire life was spent sat next to me - so I know what I'm talking about - as an opponent of the EU.
"Jeremy Corbyn, and I, and others, stayed up night after night, all through the night, trying to wreck the Maastricht treaty which established the single market. Jeremy Corbyn was an absolute opponent of the single market.
"Neither would the real Jeremy Corbyn have any truck with the idea that, in a capitalist society, it's some kind of principle that we should allow as many workers to join the queue for a declining number of jobs... and call it free movement, call it some kind of principle.
"But he is paralysed by the existence of a gigantic fifth column inside his party which sabotaged his relaunch earlier this week. They could not wait to get into the lists with their allies in the so-called mainstream media to rubbish and ridicule him."
Galloway also said the string of departures from the Labour party are meant to trigger by-elections which will force Corbyn to stand down.