Alistair Dutton, of the Scottish Catholic International Aid Fund (Sciaf), said British interventions have served to strengthen opposition forces and prolong the conflict, now approaching its six-year anniversary.
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 made the remarks during a visit to Lebanon to see how cash raised by Sciaf is helping the 1.5 million Syrian refugees who have fled there since 2011.
His comments come as Syrian government forces, backed by Russian allies, have taken nearly all rebel-held parts of Aleppo amid reports of civilian killings.
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.”
He 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.
“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.”
The UK government has provided “non-lethal” equipment and political support to moderate Syrian opposition groups in the region.
Dutton said such intervention was a “pure ego trip for people who say something must be done”.