Peter Ford, who served in Damascus between 2003 and 2006, said that Britain threatened to "stoke the fires of war" after defense secretary Michael Fallon's announcement that British troops are to be redeployed in the region.
The decision is believed to have come in response to a request from the Pentagon.
The United States abandoned a similar effort in October 2015, after it emerged that only a handful of fighters had been trained, with some surrendering to al-Qaeda affiliate Jabhat al-Nusra on crossing the Syrian border.
"I would advise the British government to think again, before it's too late. Do not be so subservient to Washington. We should use our own judgement and not allow ourselves to be dragged into dubious moves by the Pentagon," Peter Ford told Sputnik.
"The US suspended their own program a year ago because it was such a dreadful failure that they only managed to put five individuals back into Syria to carry out what they were trained to do. That program has begun again in July with no signs that it will be any more successful," he added.
Around 20 military personnel, drawn from a number of units, will be sent to Jordan and Turkey where the training will take place. The numbers are expected to be increased in the future.
The deployment comes at a time of heightened international rivalry for influence in the ongoing civil war in Syria. The Syrian regime, with the support of Russia, is staging an offensive in east Aleppo, sparking a humanitarian crisis, while attempts to retake the Daesh-stronghold of Raqqa are said to be imminent.
Ford says that Britain cannot be sure that the fighters that it's training will solely be fighting Daesh.
"The first and most obvious question is how we can be sure that these new trained elements will not just simply join the war in Syria — not to fight Daesh as Mr. Fallon claims — but to join their rebel partners in operations all over Syria?" he asked.
"If that's the case then we will just be stoking the fires of war, adding to this dreadful dynamic of outside interference leading to further outside interference and prolonging the agony of the people of Syria.
"The British government claims that it's for the purpose of fighting Daesh but there is no guarantee that the real purpose is not fight for more forces to fight the Syrian government. It must surely be considered more likely that these forces will join in the civil war."
However, Defense Secretary Michael Fallon has defended the decision to deploy troops, insisting that trainees will be screened before taking part.
He said: "All volunteers from the moderate opposition will be subject to strict vetting procedures and will receive training in International Humanitarian Law.
"Trainees will be security and medically screened prior to the start of training and will be assessed during and monitored after training."