Labour leader Jeremy Corbyn has promised to never approve of UK troops being sent to fight abroad without the backing of the UN, an organisation his new shadow chancellor once derided as a "puppet of imperialist powers". John McDonnell made the speech, uncovered by IBTimes UK, at an anti-war rally in 2007.

At the meeting the Labour MP also outlined his support for a so called Ministry of Peace, which would replace the Ministry of Defence. McDonnell explained that he had worked with Dennis Kucinich, a former Democrat congressman, on the "Departments of Peace" initiative.

"People thought this was George Orwell or something. What we wanted to do is no longer have ministries of war or ministries of defence, but to have ministries of peace so that our government and the US government signs up to becoming peace makers in the world, rather than warmongers as they are at the present," he said.

"We want to also take forward the debate on world governance – how we transform the UN from being a puppet of imperialist powers into a democratic organisation that represents the world's people."

The 64-year-old was interrupted in 2011 by an aptly named song while he was advocating the proposal to MPs during a debate on conflict prevention. "I do not know why my phone is going off. I apologise. I cannot turn the thing off. The song is Bruno Mars, 'I'd Catch a Grenade for You', which is bizarrely appropriate," McDonnell quipped.

His recent appointment to the top economic job in Corbyn's shadow cabinet was criticised after his 2003 comments on the IRA emerged. McDonnell reportedly caused outrage when he claimed the terrorists should be "honoured". The Hayes and Harlington MP also shares Corbyn's opposition to the UK's nuclear deterrent, Trident, and vehemently opposed the 2003 Iraq War.

Corbyn was pushed by former leadership rival Liz Kendall on his defence policies during a Sky News hustings after the left-winger warned of "mission creep" over the UK's involvement in the Syrian civil war. He said he could not envisage any circumstances in which he would deploy British troops at the moment and later explained the UN would have to back any deployment first.

The Conservatives immediately seized on Corbyn's comments around defence when he secured victory and claimed Labour had become a "serious risk" to the UK's economic and national security.

"Whether it's weakening our defences, raising taxes on jobs and earnings, racking up more debt and welfare, or driving up the cost of living by printing money – Jeremy Corbyn's Labour Party will hurt working people," said Michael Fallon, the defence secretary. "This is a very serious moment for our country – the Conservatives will continue to deliver stability, security and opportunity for working people."