The leader of Labour's largest trade union backer has launched a scathing attack against the party's MPs "plotting" to undermine Jeremy Corbyn. Len McCluskey, the general secretary of Unite, urged the Labour leader's critics to "stop the sniping and get behind Corbyn".

The union chief, who represents more than 1.4m members, issued the plea to Labour right-wingers during a speech at the University of Oxford's world famous debating society, the Oxford Union, on Tuesday 9 February.

"Some have sought to excuse their disloyalty to Corbyn by pointing to his own rebellious past on the backbenches," McCluskey declared.

"But who can seriously argue that his votes in parliament against the Iraq war, identity cards or university tuition fees now diminish his ability to lead the Labour party today? On all these issues he was not only right, I believe, he was giving voice to the views of most Labour supporters."

He added: "I'm not saying that any Labour MP should have to abandon his or her own views, or cease to articulate them within the party's democratic structures. But I am saying that this continual war of attrition is achieving nothing beyond taking the pressure off the government.

"So my clear message to the plotters is – stop the sniping, stop the scheming, get behind Jeremy Corbyn and start taking the fight to the Tories."

McCluskey, whose union backed Corbyn's leadership campaign and has donated millions to Labour, also directly addressed Michael Dugher's threat. The former shadow transport secretary said the elections in May would serve as a serious test for Corbyn's leadership credentials.

"This is a sensitive issue and I am not a supporter of going back to mandatory re-selection or other changes designed to intimidate or undermine Labour MPs. But I also believe that we need to issue a clear warning to those who are advocating the PLP being used as a lever to force Corbyn out," the Unite chief argued.

"The bizarre plans outlined by Joe Haines and pollster Peter Kellner, the call to arms by Damian McBride in his Times article and the ludicrous 99 days' notice given by Michael Dugher to the arch-Tory Mail on Sunday – all have to be dismissed with disdain by any real Labour supporter.

He added: "If the Labour MPs want something constructive to do, then start working out policies and ideas that might help attract voters back to Labour. The leadership election revealed just how much the New Labour faction had run out of political impetus.

"They offered no answers to the big questions of inequality, economic management, and 21st century social justice. There were certainly no big ideas from what were dubbed the 'mainstream candidates' during the last leadership election."

The latest opinion poll from YouGov for The Times, of more than 1,600 people between 3 and 4 February, put Labour 10 points behind the Conservatives (29% versus 39%), with Ukip on 18%, the Liberal Democrats on 6% and the Greens on 4%.