David Cameron has suggested Jeremy Corbyn is not fit to be Prime Minister as the Conservatives step up their attack on the veteran left-winger.

The Prime Minister also accused the Islington MP expected to named Labour's new leader of wanting to take Britain back "to the days of Michael Foot and Arthur Scargill".

Mr Corbyn has criticised the Tory Government’s decision to launch a deadly drone strike against two British jihadists in Syria.

Hitting back, Mr Cameron said that politicians who cannot take "difficult" decisions should not be Prime Minister.

He said: “One of the greatest responsibilities of being Prime Minister and leading our country is doing everything you can to keep our country and our people safe.

"And we have to be clear that these are not my words, these are the words of the leaders of our security intelligence services: we face an Islamist extremist threat in our country that is bigger than any one we have faced before, and therefore we have to take action right across the board.”

He added: “At the end of the day the Prime Minister has to take difficult decisions to keep the country safe.

"That is part of the job description and if you can’t make those decisions, you shouldn’t do the job.”

The Conservatives spent much of the summer maintaining a low profile allowing the Labour leadership candidates to slug it out in public between themselves.

However, in a marked change of tack the Prime Minister launched not once but two broadsides against Mr Corbyn yesterday.

In the second he said he feared the Labour frontrunner would to reopen arguments over nationalisation, nuclear weapons, taxation and union laws.

However, reports suggest that Mr Cameron has also privately urged Conservatives not to go to far in their attacks on the Islington MP

Many Tory MPs are openly thrilled at the prospect of Mr Corbyn’s election.

They believe his left-wing views, including opposition to Trident and descriptions of Hamas and Hezbollah as “friends”will alienate middle England.

The party already believes that Labour has lost a lot of ground during its long leadership battle.

During that time the Tories have consciously tried to rebrand themselves within traditional Labour territory, as the party of the workers.

Mr Corbyn's opponents Liz Kendall, Yvette Cooper and Andy Burnham have all but conceded defeat in the bruising race.

One source close to the Burnham camp suggested that Mr Corbyn was within the “margin of error” of winning the necessary 50 per cent of votes in the first round today, in what would be a stunning victory.

If Mr Corbyn is elected he and David Cameron will face each other across the Commons on Wednesday.

Last night Scotland's sole Labour MP said that whoever was elected today it would mark the start of a new chapter for Labour north of the border.

Edinburgh MP Ian Murray said: "In Scotland, we have seen the effect that new leadership can have – Kezia Dugdale has started the process of renewing our party and will be offering fresh ideas and a new generation of candidates at next year’s elections.

“No matter who wins, Scotland will be at the top of their agenda. The route to victory in the next UK General Election in 2020 must run through Scotland and we have a big job ahead of us in order to win back support ahead of the Scottish elections next year.

“Despite our defeat, we have to remember that over nine million people voted Labour in May – 700,000 of them in Scotland. We owe all of them an effective opposition to the Tories over the next five years. And we must build from there for the next Labour majority. We begin that work.”