Boris Johnson has caused more trouble for Number 10 and David Cameron by claiming the prime minister's estimate of 70,000 moderate Syrian rebels could be "exaggerated". The Mayor of London made the admission as he urged the Conservative government to work with Russian President Vladimir Putin and Syrian leader Bashar al-Assad to defeat the Islamic State (Isis).

The figure of 70,000 non-Islamist fighters formed a central part of the prime minister's argument for UK military intervention in Syria ahead of a vote on the issue. MPs then backed Cameron's proposal to bomb IS (Daesh) targets in the Middle East nation and the Royal Air Force has since carried out operations in the area.

But Johnson, who voted for the motion as the newly-elected MP for Uxbridge and South Ruislip, wrote in his weekly column in The Daily Telegraph: "We have the estimated 70,000 of the Free Syrian Army (and many other groups and grouplets); but those numbers may be exaggerated, and they may include some jihadists who are not ideologically very different from al-Qaeda."

The figure stirred controversy in the House of Commons as Tory MP Julian Lewis, who is also the chair of the Defence Committee, described the estimate as a revelation. He said: "The suggestion that there are 70,000 non-Islamist credible ground forces I have to say is a revelation to me and I suspect most other members in this house."

But Columb Strack, a senior Middle East analyst at IHS, told IBTimes UK that the number depended on where you drew the line on who you considered to be an Islamist. "I'd say the 70,000 figure is roughly in the right ballpark in terms of the number of rebels that would be willing to enter into a dialogue including Western and other countries over the future of Syria," Strack said.

"However, many of these must still be considered Islamist in that their ultimate goal is to establish a Sunni Islamic state. Practically all of the key players among the rebel factions in northern Syria (e.g. Ahrar al-Sham, Jaysh al-Islam, Jabhat al-Nusra) are Islamists to varying degrees. Groups operating in the south of the country, particularly those under the Free Syrian Army's Southern Front (about 35,000 fighters) are generally more secular."