The collision between French and British nuclear submarines was a hugely improbable one-off, but it was no surprise that they did not detect each other, experts said Monday.

For one nuclear-powered, nuclear arms submarine to collide with another one in the middle of an ocean was unprecedented and sheer bad luck, they said.

Experts played down the chances of a nuclear disaster, saying the subs were designed to be robust and a collision was unlikely to trigger a nuclear reaction.

Stephen Saunders, the editor of Jane's Fighting Ships and a retired commodore, nonetheless called it a "very serious incident".

"As far as I am aware, it is the first time that the submarines of two friendly nations have been involved in such an accident," he said.

"No doubt there are a number of technical issues to be investigated, but the root of the problem appears to be procedural. These submarines should not have been in the same place at the same time.

"Even if two submarines do find themselves in the same area, it is still bad luck to have run into each other -- i.e. to be in the same place at the same depth."

Lee Willett, head of the maritime studies programme at the Royal United Services Institute (RUSI), a defence and security think-tank, said that the incident was a complete one-off.

"These are the strategic crown jewels of a nation. The whole purpose of a sea-based nuclear deterrent is to hide somewhere far out of sight and out of mind," he told AFP.

"Given that there are a very, very small number of SSBNs (Ship Submersible Ballistic Nuclear) in the world, the chances of two SSBNs being in the same bit of water at the same time is unprecedented.

"The fact that they couldn't hear each other is actually less surprising."

Saunders explained: "The modus-operandi of most submarines, particularly ballistic-missile submarines, is to operate stealthily and to proceed undetected.

"This means operating passively (i.e. not transmitting on sonar) and making as little noise as possible.

"While in parallel much effort has gone into improving the capability of sonars to detect other submarines, detection was clearly made too late or not at all in this case."

Willett added: "Submarines don't go around advertising their position by pinging away with their sonar. It's very hard to hear stuff under water becasue of all the ambient noise.

"SSBNs listen passively using their sensing equipment but if you're listening for something that's making no noise, you can't hear it."

The RUSI expert said that despite the close NATO and European Union ties between Britain and France, the two countries would be very reticent to share information on what their nuclear submarines were up to.

"Despite how close these relations are, they are the ultimate tools of national survival in the event of war. Therefore it's the very last thing you would share with anybody."

However, he added: "This may raise the question of agreeing on waterspace management issues: you go there, we go there. Those agreements may exist already.

"Both nations have now lost a big part of their SSBN capability for a period of time.

"Clearly it's not something they would be keen to repeat."