General Sir David Richards' bid for 100,000-strong force had support from US heavyweights but was shelved as too risky

The British military drew up a secret plan two years ago to train a 100,000-strong Syrian rebel force aimed at toppling president Bashar al-Assad.

The plan was the brainchild of General Sir David Richards, then chief of staff. The military regularly draws up contingency plans for all kinds of scenarios, but this one was considered more seriously than most and widely circulated – including to Downing Street and senior US military staff. It was shelved as being too risky.

The Ministry of Defence declined to confirm or comment. But according to the BBC, Richards proposed an international coalition to vet and train an army of moderate Syrian rebels at bases in Turkey and Jordan for about a year.

It would then march on Damascus, with air cover from western forces and Gulf allies. The plan was drawn up at a time of strong support in the UK and US governments for intervention in the Syrian civil war by arming and training the rebels. Among prominent supporters in the US were then secretary of state Hillary Clinton, then defence secretary Leon Panetta and David Petraeus, who served as the head of the CIA as well as the head of US central command and the coalition forces in Afghanistan.

But Barack Obama, who had devoted much of his presidency to getting US troops out of Iraq, opposed the prospect of becoming caught up in another Middle East conflict – particularly arming rebels from hardline militant groups hostile to the US.

Any chance of British involvement was finally scuppered when MPs voted against action in August last year.

Lord Richards' proposal was aimed at ending a civil war estimated to have cost more than 100,000 lives.

Two years on, Obama – worried about increased instability in the region – has had a rethink as hardline militant group the Islamic State of Iraq and the Levant is taking over swaths of Iraq and parts of Syria. He has asked Congress to approve $500m (£291m) in funding to train Syrian rebels.

Monzer Akbik, from opposition group the Syrian National Coalition, told the BBC: "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.

"The international community did not intervene to prevent those crimes and at the same time it did not actively support the moderate elements on the ground."