The British Divers Marine Life Rescue group said a first group of boats had begun "gently moving" the deep-diving mammals from Loch Long towards the mouth of the River Clyde.

Loch Long, north of the western city of Glasgow, is near the Faslane naval base, which is home to Britain's fleet of nuclear-powered submarines.

Thousands of armed forces personnel from the UK, NATO and other international units are due to take part in the Joint Warrior military exercise in the area from next week.

The BDMLR said it was working with locals and the UK Ministry of Defence to monitor the whales, and rescuers had formed a barrier with boats to prevent them turning in the wrong direction.

"We recently became aware that a significant military exercise is due to begin next week, and as whales are particularly sensitive to underwater sound, have been concerned about the effect it may have on the animals," the group said.

"Therefore in consultation with colleagues at Whale and Dolphin Conservation and Hebridean Whale and Dolphin Trust, the decision has been made to attempt to herd the animals out using a number of boats in formation to get them back to open sea."

Northern bottlenose whales are normally found off the edge of the continental shelf to the west of the UK and Ireland.

It is unusual for them to be seen in coastal waters, according to the rescue group.

Around six boats were involved in the rescue on Thursday.

The coastguard in Greenock, on the southern shore of the loch and the mouth of the Clyde that runs through Glasgow to the sea, has asked passing vessels to move at "slow speed" and with "particular caution" so as not to disrupt the operation.