HMS Sutherland acted in international waters
Charles Shoebridge was speaking as the ship, MV Nisha, was searched in the English Channel by the Anti-Terrorist Branch and the Royal Navy on Friday afternoon.
The major security alert was triggered by intelligence that the ship - travelling from Mauritius in the Indian Ocean - might be carrying terrorist paraphernalia.
A preliminary search found nothing but further more detailed searches are expected to take several days.
Intelligence tip-off
It is not being confirmed whether this operation was linked to the 11 September terror attacks on America.
But Mr Shoebridge, a former anti-terrorist intelligence officer, said it was highly significant that police decided to stop the ship before it had reached British waters.
"This suggests that what was possibly on board was not something simply to be seized but something that may have posed a significant threat to the population of Britain," he said.
He was convinced the operation was the result of a tip-off and based on intelligence work in the aftermath of the 11 September attacks.
Mr Shoebridge said: "It is extremely unusual for anti-terrorist officers to board ships in international waters."
He said logistically it would have been far simpler to have waited until the ship docked in the UK.
Vehicles would have been at the ready if arrests were made and police officers would have been on hand to thoroughly search the ship.
In terms of legislation, it would have been simpler to wait for the ship to reach UK waters and fall under Britain's jurisdiction although an operation would fall within international law.
Changed attitudes
Security forces already have a proven track record of intercepting terror-related cargo on the High Seas.
In the 1980s for example a consignment of 150 tons of weapons and ammunition destined for the Provisional IRA was intercepted en route from Libya on the French coaster Eksund by international authorities.
Mr Shoebridge said what had changed since 11 September was not the measures taken to tackle security threats on the high seas but attitudes to taking those precautionary actions.
"There is a heightened vigilance. There is perhaps more of a willingness to enforce the law as it stands and reluctance to take any risks now," he said.
He said that in the past they may have assessed the situation more thoroughly taking into account factors such as the cost of the operation and likelihood of compromising a criminal case.
It is uncertain as of yet whether this incident will turn out to be a false alarm or not.
"But now with the threat levels so high they are not going to take any chances," said Mr Shoebridge.