Britain's policy on Syria has been "wrong every step of the way", a former UK ambassador to the country has said.
Peter Ford said the UK had made matters worse by not putting troops on the ground and instead encouraging rebel groups to mount a doomed campaign.
The situation had led to hundreds of thousands of civilian casualties, which could have been foreseen, he added.
The Foreign Office said removing the Assad regime was the only way to end the suffering of the Syrian people.
Mr Ford, who was the ambassador in Damascus from 2003 to 2006, said the UK should have put forces on to the battlefield or refrained from encouraging the launch of the opposition campaign.
In 2013, possible military action against the Assad regime in Syria to deter the use of chemical weapons was rejected by MPs,
Two years later, the UK launched air strikes targeting so-called Islamic State in Syria after receiving Parliament's backing.
The government motion authorised air strikes "exclusively" against IS in Syria - but not deploying British troops on the ground. Speaking at the time, Mr Ford said the value of strikes would be "piddling" and warned of "inevitable blowback on our streets"..
Speaking on the Today programme on Friday, Mr Ford said: "We have made the situation worse.
"It was eminently foreseeable to anyone who was not intoxicated with wishful thinking. The British Foreign Office, to which I used to belong, I'm sorry to say has got Syria wrong every step of the way."
On Thursday, the Syrian army regained full control of Aleppo - which has been a key battleground in the civil war between government forces and rebel groups.
It is a notable victory for President Bashar al-Assad in the war which began when the uprising against him began in 2011.
Mr Ford said the president's government should be given "a little credit" for a "relatively peaceful" end to the siege in Aleppo.
He told the BBC government forces would now need to strengthen their hold on the city and defend it against possible counter-attacks.
'Blood on its hands'
The former diplomat criticised the Foreign Office for saying President Assad's demise was imminent at the beginning of the war and for predicting he would lose power quickly.
He said the department also said the opposition was dominated by "these so-called moderates, that proved not to be the case".
The BBC's Middle East editor, Jeremy Bowen, said the nature of the opposition side had "changed" over the last five years of war.
He said: "They were a different type of people than the way they are now. The Jihadist side has become much more prominent."
Mr Ford also said: "Now they are telling us another big lie, that Assad can't control the rest of the country. Well, I've got news for them; he is well on the way to doing so."
The West has condemned the bombing in Aleppo, yet similar attacks were being carried out in the Iraqi city of Mosul and in Yemen without the same amount of criticism from Britain, he claimed.
He went on: "We don't talk about atrocities; we don't talk about war crimes, although they are indisputably being committed in both those theatres."
The former ambassador said "we will be lucky" if those campaigns ended in the evacuation of civilians and rebel fighters on green buses, as was the case in Aleppo.
The Foreign Office said a political solution and transition away from Assad were the "only way to end the suffering of the Syrian people".
A spokesman added: "The Assad regime has the blood of hundreds of thousands on its hands. There is no way it can unite and bring stability to Syria.
"The UK has pledged more than £2.3 billion to support those affected by the Syrian conflict and sought to reduce the suffering with every diplomatic lever at our command."