Lord West unleashed a barrage of criticism against French threats to blockade the harbour and cut off the island’s electricity supply, branding it more a “19th Century way of behaving rather than 21st Century”.
He appealed for “sense” to prevail and for the situation to be de-escalated.
But he warned that the Navy could arrest French fishermen and impound their boats if the stand-off dramatically worsens.
He stressed that the Navy ship commanders would be wary of triggering clashes with around 70 French vessels massed off the island, in a row over fishing rights, and would have instructions about rules of engagement.
“They no doubt have on board Royal Marine boarding parties and we are used to boarding foreign vessels and indeed UK vessels,” he told Times Radio.
“We have done that over many years, if you go back to the Icelandic cod wars.
“So they will be working very hard to try to defuse any situations.
“But you can’t allow the vessels of another country to blockade one of our ports.
“In the final analysis, if they actually did block it, then we would have to do something to unblock it.”
The former First Sea Lord explained how the Navy would respond to such a task.
“If they wanted to move a couple of fishing boats, you would no doubt be boarded by Marines and you would arrest the people involved and then hand them over to the local police to be dealt with,” he said.
“After you have arrested some, their boats are impounded and that’s normally the thing that makes fishermen be very careful.
“If you lose all your fishing gear and your boat, that is an extremely expensive and catastrophic thing so they are normally very wary about that.
“But let’s hope we don’t get to that level.
“It’s absolutely extraordinary that we have got to this level so quickly.”
Lord West, who was security minister during Gordon Brown’s administration, added: “I would hope that sense will prevail and people will take a step back and actually look at the details of this deal and finally of course, it’s up to getting lawyers involved if they are disputing the details of the deal.
“It’s quite extraordinary to make threats of blockading, of cutting off power supplies, that really is rather sort of a 19th Century way of behaving rather than a 21st Century way of behaving.”