A secret plan to train a 100,000-strong rebel army to wage war on Syria's President Bashar Assad was drawn up by a leading British general, according to a report.

The idea - developed two years ago by Lord Richards, who was then Chief of the Defence Staff - was considered by David Cameron and the National Security Council but they ultimately decided against carrying it out.

The plan, revealed on the BBC's Newsnight programme, called for Syrian rebels to be removed from their home country, put through a vetting process, given training and equipment by an international coalition and then sent back after a year.

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They would have invaded Syria with air cover provided by the West and allies in the region in a “shock and awe” campaign modelled on the 2003 invasion of Iraq.

The Government did not respond to Newsnight when it was asked to comment.

Tens of thousands of people have been killed and millions have become refugees during the three-year civil war, which has seen chemical weapons used by the Assad regime, according to the rebels and others.

Video: The war in Syria

Monzer Akbik, spokesman for the Syrian National Coalition, an opposition alliance, said: “The international community did not intervene to prevent those crimes and at the same time did not actively support the moderate elements on the ground.

"A huge opportunity was missed and that opportunity could have saved tens of thousands of lives actually and could have saved also a huge humanitarian catastrophe."

Professor Michael Clarke, of the Royal United Services Institute think tank, said the chance had been missed to train "an anti-Assad force that would have real influence in Syria when he is removed, as he will be".

Shape Created with Sketch. In pictures: Syria elections Show all 20 left Created with Sketch. right Created with Sketch. Shape Created with Sketch. In pictures: Syria elections 1/20 Syria elections Protestors who oppose Syrian President Bashar Assad carry a banner during a demonstration at Kafr Nabil town in Idlib province, northern Syria AP 2/20 Syria elections A banner with a picture of Syrian President Bashar Assad with a slogan reading in Arabic, " we build it together" hangs on a destroyed building in the Homs neighborhood of Khaldiyeh 3/20 Syria elections Syrian Prime Minister Wael Halqi speaking to media after he casted his ballot in the presidential elections at a polling station in Damascus 4/20 Syria elections Syrian defence minister Fahd Jassem al-Freij (C) casting his ballot in the presidential elections at a polling station in Damascus 5/20 Syria elections Employees emptying the poll boxes following to the closure of polls at midnight local time in Damascus 6/20 Syria elections Syrian students, one wearing a T-shirt bearing a portrait of President Bashar al-Assad, prepare to cast their votes in the country's presidential elections at a polling station in the Baath University of Homs, north of Damascus 7/20 Syria elections A Syrian student shows her ink-stained thumb after casting her vote in the country's presidential elections at a polling station in the Baath University of Homs, north of Damascus 8/20 Syria elections A Syrian woman rides in a car painted in the colors of the Syrian flag with President Bashar Assad's portrait in Damascus 9/20 Syria elections A man casts his ballot in Damascus 10/20 Syria elections Supporters of of Syria's President Bashar al-Assad celebrate in front of a polling centre in Damascus 11/20 Syria elections Syrian President Bashar al-Assad casts his vote at a polling station in Maliki, a residential area in the centre of the capital Damascus 12/20 Syria elections Syrian President Bashar al-Assad (L) and his wife Asma al-Assad (R) casting their votes in the presidential elections at martyr Na'em Ma'asarani School in al-Malki neighborhood in Damascus 13/20 Syria elections An old trains station is decorated with posters showing President Bashar Assad in Damascus 14/20 Syria elections A Syrian man casts his ballot at a polling site in the Um-Al Zinnar church in the Old City in Homs 15/20 Syria elections A ballot paper bearing the portraits of the three presidential candidates (LtoR) Maher Abdel Hafiz Hajjar, President Bashar al-Assad and Hassan Abdallah al-Nuri displayed at Bassel al-Assad school turned into a polling station for the presidential election in central Damascus AFP 16/20 Syria elections Syrians walk past campaign posters of Syrian President Bashar al-Assad displayed on a stall at a market in the capital Damascus AFP 17/20 Syria elections Syrian men pretend they are casting their votes during a mock election calling for the "criminal" Syrian President Bashar al-Assad to the stripped of his Syrian nationality in the mostly rebel-held city of Aleppo 18/20 Syria elections Supporters of Syrian President Bashar Assad hold his portraits and wave Syrian flags during a demonstration in support of his candidacy for presidential election in the costal city of Tartous AP 19/20 Syria elections A Syrian man shows a ballot paper bearing the portraits of the three presidential candidates (LtoR): Maher Abdel Hafiz Hajjar, Bashar al-Assad and Hassan Abdallah al-Nuri AFP 20/20 Syria elections An electoral worker holds a voting card at a polling center in Damascus EPA 1/20 Syria elections Protestors who oppose Syrian President Bashar Assad carry a banner during a demonstration at Kafr Nabil town in Idlib province, northern Syria AP 2/20 Syria elections A banner with a picture of Syrian President Bashar Assad with a slogan reading in Arabic, " we build it together" hangs on a destroyed building in the Homs neighborhood of Khaldiyeh 3/20 Syria elections Syrian Prime Minister Wael Halqi speaking to media after he casted his ballot in the presidential elections at a polling station in Damascus 4/20 Syria elections Syrian defence minister Fahd Jassem al-Freij (C) casting his ballot in the presidential elections at a polling station in Damascus 5/20 Syria elections Employees emptying the poll boxes following to the closure of polls at midnight local time in Damascus 6/20 Syria elections Syrian students, one wearing a T-shirt bearing a portrait of President Bashar al-Assad, prepare to cast their votes in the country's presidential elections at a polling station in the Baath University of Homs, north of Damascus 7/20 Syria elections A Syrian student shows her ink-stained thumb after casting her vote in the country's presidential elections at a polling station in the Baath University of Homs, north of Damascus 8/20 Syria elections A Syrian woman rides in a car painted in the colors of the Syrian flag with President Bashar Assad's portrait in Damascus 9/20 Syria elections A man casts his ballot in Damascus 10/20 Syria elections Supporters of of Syria's President Bashar al-Assad celebrate in front of a polling centre in Damascus 11/20 Syria elections Syrian President Bashar al-Assad casts his vote at a polling station in Maliki, a residential area in the centre of the capital Damascus 12/20 Syria elections Syrian President Bashar al-Assad (L) and his wife Asma al-Assad (R) casting their votes in the presidential elections at martyr Na'em Ma'asarani School in al-Malki neighborhood in Damascus 13/20 Syria elections An old trains station is decorated with posters showing President Bashar Assad in Damascus 14/20 Syria elections A Syrian man casts his ballot at a polling site in the Um-Al Zinnar church in the Old City in Homs 15/20 Syria elections A ballot paper bearing the portraits of the three presidential candidates (LtoR) Maher Abdel Hafiz Hajjar, President Bashar al-Assad and Hassan Abdallah al-Nuri displayed at Bassel al-Assad school turned into a polling station for the presidential election in central Damascus AFP 16/20 Syria elections Syrians walk past campaign posters of Syrian President Bashar al-Assad displayed on a stall at a market in the capital Damascus AFP 17/20 Syria elections Syrian men pretend they are casting their votes during a mock election calling for the "criminal" Syrian President Bashar al-Assad to the stripped of his Syrian nationality in the mostly rebel-held city of Aleppo 18/20 Syria elections Supporters of Syrian President Bashar Assad hold his portraits and wave Syrian flags during a demonstration in support of his candidacy for presidential election in the costal city of Tartous AP 19/20 Syria elections A Syrian man shows a ballot paper bearing the portraits of the three presidential candidates (LtoR): Maher Abdel Hafiz Hajjar, Bashar al-Assad and Hassan Abdallah al-Nuri AFP 20/20 Syria elections An electoral worker holds a voting card at a polling center in Damascus EPA

"I think there was an opportunity two or three years ago to have become involved in a reasonably positive way, but it was dangerous and swimming against the broader tide of history … and the costs and the uncertainties were very high."

He added there was little that could now be done. "There are no good options over Syria. It is a slow-motion road accident," he said.

Extreme Islamist militants have gained strength within the rebel force, partly because they are well equipped using funds sent from like-minded people and groups in the Middle East. Isis recently moved into Iraq, taking significant territory.