A plan to train and equip a 100,000-strong rebel army to defeat Syrian President Bashar al-Assad was drawn up by Britain, then scrapped as it was deemed too risky, BBC Newsnight reported on Thursday.
The alleged proposal, put forward two years ago, was the idea of General David Richards, then chief of Britain's defense, according to the BBC.
The secret "extract, equip, train" proposal was considered by Prime Minister David Cameron and the National Security Council, as well as U.S. officials, the BBC said, citing Whitehall sources.
As part of Richards' proposal, the "substantial army" of moderate Syrian rebels would have been trained at bases in Turkey and Jordan, Al Arabiya reported.
The United Nations says some 10.8 million people in Syria need assistance, of which 4.7 million are in hard-to-reach areas, while another three million have fled during the three-year uprising against Assad.
Some believe the opportunity to carry out such an initiative has been missed.
Professor Michael Clarke, of the Royal United Services Institute think tank, told the BBC: "We have missed the opportunity to train an anti-Assad force that would have real influence in Syria when he is removed, as he will be.
"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."
Some 400 British nationals are thought to be fighting in Syria, in what could constitute a crime under UK terrorism legislation.