Labour has said there are "serious questions" to be answered over how a British man was able to flee to Syria while on bail before reportedly appearing in an IS propaganda video.
Siddhartha Dhar travelled to Syria in 2014 while on bail after being arrested on suspicion of encouraging terrorism.
There have been reports he is the masked man in so-called Islamic State's latest propaganda killings video, although the Home Secretary declined to comment on his identity in the House of Commons on Tuesday.
Shadow Home Secretary Andy Burnham said whether or not Dhar is the man in the video, there had been "a major lapse in security" and called for an inquiry into how he was able to leave the country.
Theresa May said she could not comment on an "ongoing police investigation".
Despite multiple claims to the contrary, the man behind the mask in the latest IS propaganda video has not yet been identified.
A leading candidate emerges in the push to name the 'Brit' from the latest IS killings video - but his sister and mentor say it isn't him.
Henry Dare told ITV News he had not seen his grandson for three years since his daughter, a known jihadi, took him with her to Syria.