Christian Baroness Cox joined other members of the House of Lords and church leaders to hold meetings with the Assad regime.
Tory MP Nigel Evans said at the time it "defied logic" that the group visited the country just a week after dozens were suspected killed by chemical weapons in Douma.
However, Lady Cox told Premier she questioned whether there had been a chemical weapons attack at all.
Condemning the UK's participation in air strikes, the crossbench member of the House of Lords said: "Everyone is...deeply, deeply angered by the use of missiles by the UK, the US and by France really for three reasons - one, it was illegal.
"There was no mandate and Syria is not about to attack our country, so there's no legal justification and also the timing. They actually sent the missiles the day before the chemical weapons investigators were due to investigate."
"There's also a lot of concern about the British government's continuing commitment to so-called regime change. There is no moderate armed opposition left so any regime change would just open up the situation to another Iraq, another Libya - that's the last thing they want."
Lady Cox's visit earlier this month included a meeting with the Grand Mufti of Syria, who has been accused by Amnesty International of authorising the mass hangings of prisoners in the country's Saydnaya jail.
The peer said she was not aware of the claims and "if that's the case I don't in any way condone it".
Asked if she expects the world to accept a leader that uses chemical weapons, she told Premier: "There is no proof that he [Assad] did.
"[It] seems there's no reason why he should use chemical weapons. He's winning the war military and there's no proof it was he who used them...It's very worrying that we would actually attack another country with missiles, before there's any evidence of who actually did what it was - if it was done at all.
"One's not condoning things that are uncondonable but this situation did not merit the use of missiles in that way. If you talk to Syrian people - everybody, including the church leaders - they say that the alternative would be much, much worse.
"For some, they've actually moved to appreciating the fact that it is he - with Russian help - whose helped to get Syria rid of ISIS and all the other Islamist terrorists. [ISIS is] the greatest enemy of all.
"The truth must come out and we know as Christians that only the truth will make us free and there is a lot of concern in Syria...that we are only getting one side of the story for much of the time.
"So we must find the truth - that's one reason for going, to meet the people to find their perceptions of their reality - not judging it from a foreign land.
"We should let the people of Syria decide their own future. They're a deeply civilised people and we should stop interfering in proxy-wars.
Meanwhile, Moscow's support for Syrian president Bashar Al Assad has given him free rein to perpetrate human rights atrocities, Penny Mordaunt will say at an international conference this week.
The International Development Secretary will use the gathering to appeal to Russia and Iran to use their influence to bring the Assad regime to the negotiating table in an effort to end the bloodshed in Syria.
The European Union and the United Nations will co-chair the conference in Brussels on Tuesday and Wednesday.
Listen to Baroness Cox speaking with Premier's Marcus Jones:
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