The Ministry of Defence has been forced to ask the US for help as it hunts for a suspected Russian submarine believed to be lurking off the coast of Scotland.

Two U.S Navy aircraft were this week drafted in to carry out patrols in the North Atlantic in a bid to bridge the 'gaping chasm' in Britain's anti-submarine capability.

It comes after the RAF's £4bn fleet of 'spy-in-the-sky' Nimrod surveillance aircraft was controversially scrapped in 2010, amid warning it would weaken Britain's defences.

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The Ministry of Defence has been forced to ask the US for assistance as it hunts for a suspected Russian submarine believed to be lurking off the coast of Scotland (stock image of Russian submarine)

The latest operation follows a recent surge in Russian naval and airforce activity (shown in graphic above)

Experts said the latest request for assistance raises questions about the UK's ability to adequately protect its nuclear submarines.

And Angus Robertson, the Scottish National Party's defence spokesman, said the deployment showed Britain had resorted to going to its allies with a 'begging bowl'.

The U.S P3 Orion maritime patrol aeroplanes, which are currently based at RAF Lossiemouth, were deployed on the trail of the mystery vessel this week.

The crews worked alongside the Royal Navy's anti-submarine frigate HMS Somerset, which has been operating off Scotland for a month.

The emergence of the Russian submarine is said to be linked to the alleged departure from Faslane Naval base of one of the Royal Navy's Vanguard-class nuclear submarines, which carries Trident missiles.

The base, at Gare Loch on the River Clyde, is the home of the UK's ballistic missile submarines.

Peter Roberts, a senior fellow of the Royal United Services Institute for Defence and Security Studies, told The Independent: 'HMS Somerset is a capable platform and I have no doubt that her deployment alongside these U.S Navy aircraft is related to the reported departure of a Royal Navy Vanguard ballistic missile submarine from Faslane, and the countering of any Russian deployment from over the horizon.'

The two U.S Navy P3 Orion maritime patrol aeroplanes were deployed to fill what defence experts described as a 'gaping chasm' in Britain's anti-submarine capability (stock image)

The U.S crews co-ordinated searches alongside the Royal Navy's anti-submarine frigate HMS Somerset (pictured), which has been operating off Scotland for one month (stock image)

But he added that Ministry of Defence chiefs had been 'scratching their heads' ever since Nimrod was scrapped, adding: 'It has left a gaping chasm in the UK's capabilities and left us highly dependent on co-operation from our allies.'

This week's operation follows a deployment last month by maritime patrol aircraft from Canada, France and the U.S.

The periscope was sighted in waters where British submarines would normally surface as they head into or out of Faslane.

On that occasion, it was also suggested that a suspected Russian submarine may have been trying to track one of Britain's four Vanguard-class boats.

Sources said 'visits' from Russian submarines were 'happening quite often' off the north and west coasts of Scotland.

Some say the presence of the Russian submarine could be linked to the alleged departure of the Royal Navy's Vanguard-class nuclear submarines (pictured) from Faslane naval base (stock image)

In October, Sweden mounted a search for a suspected Russian submarine thought to be operating in its territorial waters.

Sweden closed off waters and airspace off Stockholm and warned the public to stay away after the military made three sightings of the mystery vessel.

It also said it had picked up three encrypted signals from a suspected submarine it believed to be Russian.

One theory was that an experimental Russian mini-sub called the Triton NN had got into trouble on a spy mission in Swedish waters.

Although the hunt was unsuccessful, defence officials said there was no doubt that their waters had been violated by a foreign power. Russia denied that any of its forces were involved.

In November, the Royal Navy tracked four Russian warships passing through the English Channel.

On this occasion The Ministry of Defence (MoD) said that the ships had been escorted out of UK waters by Royal Navy warship HMS Tyne.

David Cameron axed the Nimrod MRA4 aerial surveillance planes in October 2010 as part of the Strategic Defence and Security Review.

The MRA4 was intended to replace the Hawker Siddeley Nimrod MR2 and would ensure that no foreign submarines could track Royal Navy vessels coming in and out of Faslane.

The MoD had signed a deal with BAE in 1996 to build 21 of the Nimrod planes, only for the contract to be reduced to 12 and then nine aircraft.

The request for foreign assistance comes after the RAF's £4bn fleet of Nimrod surveillance aircraft (pictured) was controversially scrapped under David Cameron in 2010 (stock image)

Last month, patrol aircraft from four countries were deployed after a suspected Russian submarine was sighted at Faslane naval base (pictured) where is was allegedly tracking one of the Vanguards (stock image)

At the point of being scrapped, the project was £789m over-budget and more than nine years late.

The political move prompted criticism from top military figures and MPs who warned it would weaken Britain's defences.

Even the then Defence Secretary Liam Fox admitted the move was a 'risk', but ministers pressed on, claiming it would save the MoD £2billion.

Meanwhile, Labour leader Ed Miliband this week backed replacing the Trident nuclear weapons system with the 'least-cost deterrent' possible.

The submarine-based system is up for renewal in 2016 and the issue could be a key factor in any coalition deal after the May election.

The current system involves four submarines, with one armed boat always at sea, operating out of Faslane.

A spokesman for the MoD said: 'Tough decisions had to be taken in order to rebalance the Defence budget, which included removing the Nimrod MR2 from service.

'However, maritime surveillance is provided through a combination of layered capabilities including surface ships, submarines, and air assets such as the RAF Hercules which searched for the missing yacht Cheeki Rafiki in May.