A British naval supply ship with an on-board helicopter was deployed to track the stealthy Kilo-class submarine Krasnodar and the tug Altay, the Royal Navy said in a statement.

The Russian ships had arrived from the North Sea and entered the Channel.

"Expert aircrew were prepared to pivot to anti-submarine operations if Krasnodar had dived below the surface," the statement said.

But it sailed on the surface throughout the operation, despite unfavourable weather conditions.

Near the island of Ouessant, off northwest France, the British said they handed over monitoring of the vessels to a NATO ally, without saying which one.

The British military carried out a similar shadowing operation in July, after spotting the Russian sub Novorossiysk in its territorial waters.

Defence minister John Healey announced on Monday the launch of a multi-million pound programme to improve the Royal Navy's capabilities in the face of Moscow's "underwater threats".

According to London, Russian submarine activity in British waters has increased by about a third over the past two years.

In early December, the UK and Norway signed a cooperation agreement to jointly operate a fleet of frigates to "hunt down" these submarines in the North Atlantic.