John McDonnell has stepped up his claims that public attacks on Jeremy Corbyn are being orchestrated by “the establishment” as he vowed to move Britain’s right on to a left-wing agenda.

The shadow Chancellor said in an impassioned speech that MP and media criticism of Corbyn was being “very effectively organised by the establishment” and warned against “complacency” in the final days of the leader’s battle to be re-elected.

It is the latest in a succession of defiant messages delivered by McDonnell as he portrayed the left-wing leadership as under siege. Last month he claimed a “rigged purge” of Corbynista members was taking place but this was strongly rebutted by Iain McNicol, Labour’s general secretary, who published a dossier containing examples of the type of abuse that led to people being barred from voting.

Last night McDonnell identified centrist Labour MPs, the City and elements of the media, including The Guardian newspaper, as being to blame for the “vitriol” directed at Corbyn.

“Some of that is very effectively organised by the establishment to undermine us [but] this is the opportunity we waited for. This is the opportunity some of us have been born for,” he claimed at a meeting of the Labour Representation Committee at a fringe meeting at TUC annual congress. He did not produce further evidence for the suggestion.

“This is about the establishment saying to you ‘how dare you elect a socialist as leader of the Labour Party?'”, he added.

“This is the establishment saying ‘get back in line, get back in your box’. They want to get back to a politics which is simply a rotation of the elites dominated by the City elites.”

Corbyn is the overwhelming favourite to win the leadership contest later this month. McDonnell, who has ruled himself out of standing if Corbyn were to be deposed at some point in future, also indicated an acceptance of widespread concerns from MPs that the Labour leadership has been hamstrung by mistakes in communication or messaging.

“Yes we’ve been talking, we’ve been talking to members of the PLP. If your criticisms are about our competence then tell us about it.”