Laura Kuenssberg
BBC political editor

This gives an intriguing picture of how despite the hype, even some of his senior supporters have concerns about his mission and its long term impact on Labour's fortunes. As the former Foreign Secretary, Jack Straw, who knows Corbyn well, suggested, perhaps the person who is most terrified about Jeremy Corbyn winning is Jeremy Corbyn.

Despite the clamour of the summer, there is not enough reliable data, nor understanding of the implications of the new expanded electorate to predict the result with any confidence.

And in the last couple of weeks the fizz has gone out of the Corbyn campaign and both Yvette Cooper and Andy Burnham's teams report support coming their way.

But if Mr Corbyn does win, he may well be haunted by a sense that he didn't really want the job in the first place.

Read more on Laura's blog

As we reach the end of the session, we are getting a series of varied questions on subjects relating to funding for a key road in Devon, financial support for defibrillators, problems in the UK steel industry and the future of the UK energy sector, including the Eggborough Power Station. The PM says "unabated" coal-fired stations do need to be phased out but their replacement by renewable capacity does need to be affordable. 

Labour's Jo Cox asks whether the PM has "led or followed public opinion on the refugee crisis". Mr Cameron says the UK has done the "right thing", saying it is one of the few countries to honour the UN commitment to spend 0.7% of income on foreign aid. This gets loud cheers from the Tory benches. 

Things are beginning to warm up a little bit. The PM gives a rather terse response to a question from Tory MP David Davis and surveillance and the so-called Wilson Doctrine, which prevents phone taps on MPs. It gets ironic cheers from the Labour benches. Green MP Caroline Lucas then asks about UK and global arms sales, suggesting they are fuelling conflict in the Middle East. The PM dismisses this, saying the source of the terrible events now being witnessed are the actions of the Assad regime and Islamic State who he accuses of "butchering" people. 

We are now onto general questions from backbenchers, the first one coming from Tory Rebecca Pow on farming. The next question is from Labour's Debbie Abrahams, a persistent thorn in the side of Work and Pensions Secretary Iain Duncan Smith, who calls for him to be investigated for breaking the ministerial code. She cites a "sanctions" leaflet produced by Mr Duncan Smith's department which included fabricated quotes; telling MPs that the data about the number of people on sickness benefit who have died didn't exist; and comments by Mr Duncan Smith about disabled people on Monday. She said the recently released data about the number of people who have died on sickness benefit shows that claimants are four times more likely to die than the general population. David Cameron said in response "people should look at the facts" and told her that two newspapers had retracted claims about the figures, and suggested she should read up before asking him further questions.

SNP Westminster leader Angus Robertson asks about the role of the Commons Intelligence and Security Committee, of which he is to become a member and urges the PM to provide "all relevant information" about the circumstances behind the killing of two British citizens in a RAF drone strike in Syria. The PM says he will try to do this but the committee cannot conduct operations and he will not "contract out responsibility for counter-terrorism" to anyone else. 

Image source, EPA

Labour leadership candidate Jeremy Corbyn was urged by one of his most senior supporters to withdraw from the contest - but the message was rebuffed, the BBC understands.

BBC political editor Laura Kuenssberg said the supporter told the left wing MP "a few weeks ago" he had not entered with the intention of winning.

"The message was, this has gone far enough, time to stop", she said.

Full story here

In response to a question from Sir Peter Bottomley, the PM says degrading and destroying Islamic State is in the interests not just of Syria and Iraq but "of civilisation more broadly".

David Cameron concludes the leaders' exchanges by saying that both Syrian President Assad and the self-styled Islamic State "have to go", adding that while much can be done through diplomatic and humanitarian efforts, occasionally "hard military force" needs to be used.

Ms Harman says the government must "rise to the challenge of this time" and show that it is "strong, confident and outward-looking" rather than "fearful and inward-looking". She calls for more action to tackle the underlying problems in the Middle East, such as poverty and global inequality. 

Ms Harman says that while more must be done to return economic migrants to their country of origin, it is imperative that people are not "left to drown in the Mediteranean". Ms Harman now asks about the EU-wide response ahead of summit of leaders next weekend. The EU has to "reach its own decisions", says the PM, but the UK will take its own action as a "sovereign nation", adding that he does not believe the EU's quota-based resettlement scheme will address the underlying problems.