Titanic tourist submarines have been trapped by wreckage in the past, a friend of missing billionaire Hamish Harding has said.

Colonel Terry Virts spoke to BBC Radio 4’s Today show about the ongoing search for the missing submersible, which lost contact an hour and 45 minutes into its dive, with five people onboard.

“The sub has a bunch of ways to get it to the surface... but none of those systems worked which to me, means it’s probably stuck on something,” Colonel Virts said.

“It sounds like it’s on the surface just sitting there stuck underneath Titanic. And by the way, other submarines and missions in years past have gotten stuck under the propeller or behind walls. It’s apparently a dangerous place to operate.”