The Hatton Garden gang, which included Dartford’s Brian Reader, knew they had triggered a security alarm after hearing a phone line ring, a court has heard. 

Michael Seed, 58, is said to be the final suspect in the £13.7 million raid - nicknamed "Basil" - and tasked with securing entry to the building over the 2015 Easter bank holiday weekend.

Woolwich Crown Court heard the phone line to the Hatton Garden Safe Deposit company's alarm system was cut and a 2G blocker was used to jam the mobile signal and prevent it communicating with the outside world.

Engineering and telecommunications expert Bevan Clues said he believes a device was then used to put the alarm into a "normal state" before the phone line was reconnected, and the blocker switched off, so that the monitoring service would not notice communications had ceased.

Giving evidence on Friday, Mr Clues said the alarm stopped communicating at 11.36pm on April 2 2015 but sent a final message at 12.18am the following morning.

"When communications were restored there shouldn't have been an indication given to the ARC (Alarm Receiving Centre) but that did occur," he said.

"Something has gone wrong or a mistake was made because the ARC received communication of an intruder."

Mr Clues continued: "There was a phone call into the Hatton Garden premises, which would have been an indication - a call in the middle of the night immediately after communications were restored - that something had gone wrong."

The court has heard a security guard arrived around an hour later but told his boss it was a false alarm, while burglars were still inside.

Mr Clues said a similar method was used to "defeat" the alarms during the £1 million burglary of the Chatila jewellery store in Bond Street in 2010.

He told jurors that "expertise" was needed in both raids, while a "powerful" 2G blocker had been found at Seed's flat in Islington, north London.

However, Richard Sutton QC, suggested a different theory about the Chatila burglary based on movements picked up by sensors.

"This would typically indicate someone had hidden in the alarm cupboard or under the large table in the viewing room for 26 hours," he said.

The witness said he had considered the scenario but thought it unlikely as the burglars had gained access through the lift shaft.

Seed denies two charges of conspiracy to commit burglary over the Hatton Garden and Chatila raids and one charge of conspiracy to convert or transfer criminal property.

Brian Reader, 79, John "Kenny" Collins, 78, Daniel Jones, 63, Carl Wood, 61, and Terry Perkins, who died last year aged 69, have been convicted of conspiring to carry out the burglary, while William Lincoln, 63, Hugh Doyle, 51, Terri Robinson, 38, and Bren Walters, 47, were also convicted in connection with the crime.

Jones pleaded guilty to the Chatila burglary, while Perkins died in prison before he could be tried.

Charles Matthews, 55, was convicted earlier this year of receiving stolen goods from Chatila.