The donation of one of the submarines to Serbia was initiated by the "Submariner" society from Belgrade in 2009.
The engineers were to supervise the assembly of the submarines.
The submarines now carry long-range nuclear-tipped missiles.
The submarines under construction could be converted, scrapped or sold.
The warheads and the submarines would be made in Britain.
And we build the submarines, not the [nuclear] missiles".
"The difference between the cost of building all the submarines in France and all the submarines in Australia... is a very manageable one," he said.
There are three parts to Trident – the submarines, missiles and warheads – with the submarines alone taking up to 17 years to develop.
The signals from the sonobuoys are received by patrolling aircraft; these then track the submarines.
There is no reason for Britain to press ahead with the submarines now.
Decoded messages revealed the positions of the submarines, enabling ships to avoid contact.
