Railway explosives which sparked a bomb alert and the evacuation of an entire office block were left by a railway worker by “mistake”, it has emerged.

The alarm was raised when the detonators, which can maim and cause injury, were left on the K1 bus by a railway worker on Friday, July 29.

About 100 people were evacuated from the CI Tower and a busy pub next to New Malden train station during the drama at about 6pm.

Explosive experts were also scrambled and police closed the roads.

The coin-sized devices are activated by impact and are put on tracks to warn drivers when signals fail or when engineers are working.

A Network Rail spokesman said the detonators were left on the bus by an experienced member of staff in a “genuine mistake” when he left them in his bag following a work shift rather than handing them in.

He said: “They [the detonators] fell out of his bag on the way home. When he realised his mistake he rushed back to reclaim them. He has apologised profusely to his manager.”

The spokesman said railway detonators, which are being phased out by railway engineers, could cause lacerations and burns and in extreme cases, if someone was inches away from the devices, they could lose a finger or their sight.

He said: “They are not high explosives. They are not particularly dangerous but in the wrong hands they could potentially harm people.

“They are designed for their noise rather than their high explosive power.”

The 16-storey CI Tower houses various offices including the Coombe Vale and Beverly safer neighbourhood team.

Witnesses said the evacuation was calm and orderly.

Sophie Gibbs, 22, assistant manager of Bar Malden next to the CI Tower was working when the package was discovered.

She said it was a busy Friday evening in the pub and she thought about 100 people were evacuated.

She said: “We all went out to the square and got told to move to the other side of the road where Tesco is. We had to wait about 45 minutes.

“To be fair it was rather calm – I would have thought it would be more chaotic.

“I have been here for four-and-a-half months and it’s the first time anything like this has happened. I even worked at Heathrow for a while and nothing like this happened.”

Iain Brown, who works on the ninth floor of the block for Container Investment Services, was leaving the office at the time and said he thought there were about 70 people evacuated. He said: “It was very calm, it was so calm that police were struggling to get people to move away. It was all very relaxed.”

The road cordons remained in place until about 6.50pm.

A police spokesman said: “The Met’s specialist unit that deals with explosives was called out and the suspect device was determined as railway detonators.

“The relevant railway manager was called and this incident was classified as non-suspicious.”