The explosion of a red double decker bus in central London on Sunday alarmed witnesses unaware that it was actually a stunt for a Jackie Chan action film.

The bus was travelling across Lambeth Bridge not far from Britain’s parliament when its top deck exploded with a large bang in a ball of flames.

One witness was the author Sophie Kinsella who said children in a park nearby had been “freaked” by the sight.

“I was freaked too. Looked very real,” she wrote on Twitter.

In fact it was a controlled explosion for the action film “The Foreigner”, starring actors Chan and Pierce Brosnan, which is due out later this year.

But for some it was all too reminiscent of the 7/7 attacks that killed dozens of people in London in 2005. One of those attacks targeted travellers on a red double decker bus.

John Taylor, whose 24-year-old daughter Carrie was killed in the 7/7 attacks, asked whether the stunt had been properly “thought through”.

“You can totally understand why some people would be alarmed seeing this today,” Taylor told the Daily Mirror newspaper’s online edition.

“This seems particularly insensitive.”

The bridge had been closed and the film producers had issued warnings of a controlled explosion to local residents and businesses before it was carried out.

Authorities, however, sought to reassure people afterwards.

“If you saw this on Lambeth bridge this morning don’t worry it was for a film & not a real incident,” wrote London Fire Brigade on Twitter.