Director of Catholic charity Sciaf Alistair Dutton spoke of his outrage at the Theresa May's policy on the violence that was only going to "prolong the war".
Speaking on a visit to Lebanon to see how cash raised by Sciaf is helping the 1.5 million Syrian refugees he warned the civil war had no end in sight.
He called on UK ministers to accept that the best option for stability in the country is for Syrian President Bashar al-Assad to regain control.
Dutton said: "British foreign policy is appalling towards Syria.
"It is naive to the point of being totally unrealistic and everybody I speak to in the region says we have got it wrong, and we are only making the situation worse and prolonging the civil war."
Mr Dutton added that senior figures from the region have said "the best solution for Syria is to build a future which allows for Assad to be the Syrian leader".
He stressed: "I'm not an apologist for Assad. What is happening in Aleppo is unforgivable.
"I'm as disgusted as anyone by the dreadful atrocities he has committed both in Aleppo and elsewhere.
"There is little doubt that he will continue to act in the most vicious and violent way in his quest to reassert control over the country.
"But no matter how immoral and inhumane his actions, holding to the current absolutist stance on Assad will only prolong the war and the unimaginable suffering of the Syrian people.
"His inhumanity should be contained by human-rights monitors, backed up by UN peacekeepers, which should be part of any settlement, not by the hopeless insistence that he is removed."
He said some MPs had been caught up in a "pure ego trip".
A Foreign Office spokesman said: "Assad continues to refuse to let the UN deliver supplies to the hundreds of thousands of people he is besieging.
"Even if Assad re-imposes his rule over Aleppo, millions of Syrians will remain unwilling to accept the dominance of a tyrant who has the blood of hundreds of thousands on his hands.
"The only real solution for peace and stability in Syria is an enduring political solution based on transition away from the Assad regime to a government representative of all Syrians."
