The UK Ministry of Defence plans to spend more than £600m on drones to patrol Britain's coasts from terrorists, drug smugglers and illegal immigrants.

Defence chiefs aim to spend £640m to buy eight new £80m Triton Global Hawks, unmanned spy drones that can keep a 24-hour watch on Britain's coastline from ten miles above ground – after becoming concerned about recent Russian incursions into British waters, according to The Sun on Sunday.

The report says ministers are concerned that failing to deploy spy drones is 'encouraging potential enemies'. The drones could also be used to help fight terrorism, drugs and illegal immigration.

The drones carry surveillance equipment that can detect hostile aircraft from 2,000 miles away and are fitted with powerful cameras that can zoom in on pilot's faces.

The investment comes amid savage defence cuts that have seen the Ministry of Defence budget cut by 8% in real terms in the four years since the Comprehensive Spending Review in 201. The cuts mean that 30,000 armed forces personnel will be cut by 2020, reducing the armed forces to 147,000. The cuts will include the reduction of 20,000 Army soldiers, 6,000 Navy officers and 5,000 RAF officers.

The Ministry of Defence has also scrapped plans to purchase a £4bn Nimrod surveillance maritime patrol aircraft in 2010 – leaving the Royal Navy and Royal Air Force to depend on planes borrowed from the US, Canada and France.

An MOD insider told The Sun on Sunday: "Drones are cheaper to fly and maintain and can remain on target for hours on end. We have no real idea who is entering British waters and what their intentions are."

An MOD spokesperson said in a statement: "We are considering a number of potential capability solutions."