Boris Johnson has said he is "sickened" by reports president Bashar Assad's regime killed up to 13,000 people in secrecy in a Syrian jail.
The Foreign Secretary also appeared to back away from his suggestion Assad should be allowed to run for re-election by insisting he "has no future as leader".
It comes after an Amnesty International report claimed the victims, many of them civilians opposed to the Government, were hanged in Saydnaya Prison in five years.
The killings took place at least weekly between 2011 and 2015 where groups of up to 50 were removed from their cells and killed, according to the investigation.
Evidence was drawn from interviews with witnesses including judges, officials and former guards at the prison north of Damascus.
Mr Johnson tweeted: "Sickened by reports from Amnesty International on executions in Syria. Assad responsible for so many deaths and has no future as leader".
His comments indicated a hardening of his position since he told the International Relations committee last month that Britain might have to "think afresh" about how to handle the Syrian crisis and had failed to live up to its mantra that Assad must go.
Mr Johnson told the committee a "democratic resolution" should be looked at and that could include an election that was "properly supervised by the UN" and gave displaced Syrians a vote.
Asked if that meant allowing Assad contest the vote, he replied: "Yes."
But at Tuesday's Cabinet meeting, Mr Johnson stressed Assad cannot take control of a democratically elected government in Syria.
Discussing the meeting, a spokesman for Prime Minister Theresa May told a regular Westminster briefing: "The Foreign Secretary stressed that Britain will continue to look for a political settlement in Syria, that we don't believe that Assad can govern the country or take control of its democratically elected government, pointed out that we will also continue our efforts to tackle Daesh in the region and also spoke about the appalling humanitarian crisis that still is affecting Syria despite the fragile ceasefire still holding."
Asked if Mr Johnson's call for Assad to go was at odds with his comments to the committee, the spokesman said: "He was very clear... I think there's been some slight misinterpretation of what he said.
"He was very clear that government policy is Assad has to go and that remains the case, we don't see that Assad could run a democratically elected government in Syria."
The spokesman added: "The Government's position on Assad hasn't changed."
Commenting on the report on mass killing, Lynn Maalouf, the deputy director for research at Amnesty's regional office in Beirut, said: "The horrors depicted in this report reveal a hidden, monstrous campaign, authorised at the highest levels of the Syrian government, aimed at crushing any form of dissent within the Syrian population."