Labour MPs are urging Owen Smith to challenge Jeremy Corbyn over his past associations with the IRA in the battle for the Labour leadership.

Nervous MPs also want him to be more combative in election hustings.

Yesterday, the two men were embroiled in an explosive row after Mr Smith suggested that the West should negotiate with Islamic State (IS) terrorists.

Read more: Owen Smith 'unfit for leadership' after suggesting IS should join peace talks

Mr Corbyn’s camp described the comments, during a head-to-head on the BBC's Victoria Derbyshire programme, as "hasty and ill-considered".

The Conservative MP Johnny Mercer, a former Army officer, said that the remarks demonstrated Mr Smith’s "unfitness for leadership".

Mr Smith’s aides later made clear that IS would have to renounce violence and commit themselves to a peaceful settlement before any negotiations could take place.

Mr Corbyn is the frontrunner in the race, although he suffered a setback last week when judges ruled that 130,000 new Labour members could not vote in the election.

Most had been expected to back the Islington MP.

Read more: Owen Smith 'unfit for leadership' after suggesting IS should join peace talks

One English Labour MP of Mr Smith: “I think he could still win this, and I want him to win this, but he needs to attack Corbyn more in the debates.”

Another said: “I want Owen to start tackling (Corbyn) about things like the IRA. After all he used to work in Northern Ireland, he knows what he is talking about. ”

Mr Corbyn has faced criticism for bringing members of the IRA to the House of Commons during the 1980s.

Last year he was criticised by families of victims of the terror group, after he appeared to dodge explicitly condemning the IRA in an interview with BBC Radio Ulster.

Earlier this year, Mr Corbyn defended his actions over Northern Ireland, saying: "Yes, I did make myself very unpopular with some people by a preparedness to reach out to the Republican tradition in Ireland, to say ultimately this war is unwinnable by either side, there is never going to be a military (answer) - therefore there has to be a political dialogue.

"At the same time, secretly, the British government was also engaged in that and then eventually in 1994 we got the first ceasefire."

Mr Smith referred to his time as a special adviser to Northern Ireland secretary Paul, now Lord, Murphy during the Derbyshire debate.

Read more: Owen Smith 'unfit for leadership' after suggesting IS should join peace talks

On IS, he added: "Ultimately all solutions to these sorts of international crises do come about through dialogue so eventually, if we are to try and solve this, all of the actors do need to be involved.

"But at the moment (IS) are clearly not interested in negotiating."

On the same issue, Mr Corbyn said: "They are not going to be round the table. No."

One of Mr Smith’s aides said: "Owen's experience of helping to bring about peace in Northern Ireland is that eventually all parties who truly believe in delivering peace have to be around the table.

"In the Middle East at the moment that clearly doesn't include - and may never include (IS)".

A spokesperson for Jeremy Corbyn said IS could not be part of the negotiations.

"Instead, its sources of funding and supplies must be cut off."

There was a lighter moment during the debate, when Mr Corbyn failed to recognise television stars Ant and Dec.

Meanwhile, a new poll again suggests Labour supporters are happier with the Conservative leader than with their own.

IPSOS Mori found 45 per cent of Labour backers were satisfied with the new Prime Minister, compared to the 39 per cent who said that they were happy with Mr Corbyn’s performance.