More than £100 million is being spent on upgrading Portsmouth naval base because the harbour is too shallow to accommodate the two biggest warships ever built for the Royal Navy.

The complex and expensive operation is needed to deepen the harbour by 5ft so that the giant aircraft carriers HMS Queen Elizabeth and HMS Prince of Wales will be able to berth there.

Over the next seven months, about three million cubic metres of clay, sand and gravel will be excavated from the approach channel, the harbour itself and the berth – covering an area roughly equivalent to 200 football pitches.

More than £100 million is being spent on upgrading Portsmouth naval base because the harbour is too shallow to accommodate the two biggest warships ever built for the Royal Navy. Above, HMS Queen Elizabeth sighted in the River Forth earlier this year

Over the next seven months, about three million cubic metres of clay, sand and gravel will be excavated from the approach channel, the harbour (pictured) itself and the berth

Major work will also be carried out on reinforcing the jetty with 3,300 tons of steel, installing bespoke navigational lights and enlarging gangways.

A barge-mounted crane, called Strekker, is already undertaking the preparatory task of removing underwater debris. Its haul so far has included concrete blocks, discarded cables, chains and several rusting anchors, some of which are thought to be at least a century old.

The dredging ships Freeway and Crestway, each with two 3ft-diameter pumps, will suck sediment from the sea floor. Project manager Paul Simmonds said: ‘The pumps work like vacuum cleaners. They suck the sand into hoppers and then disgorge it at a Government-approved offshore disposal site near the Isle of Wight.’

The dredgers, each with a crew of about 15, are operated by Dutch company Boskalis Westminster, which in June won a £31million contract from the Ministry of Defence to deepen Portsmouth harbour.

The complex and expensive operation is needed to deepen the harbour by 5ft so that the giant aircraft carriers HMS Queen Elizabeth and HMS Prince of Wales will be able to berth there

The massive vessel will be as long as twenty eight London buses which is three times bigger than Britain's previous biggest aircraft carriers

Military experts say the two new aircraft carriers, built in sections around the country and assembled at Rosyth on the Firth of Forth, will form the centrepiece of Britain’s naval capability.

The vessels are 920ft long – equivalent to 28 London buses parked end to end – and are three times bigger than previous carriers.

Taller than Nelson’s Column and displacing 65,000 tons, the carriers will provide the Armed Forces with a massive operating base that can be deployed worldwide. Prime Minister David Cameron has called them ‘an investment in British security, British prosperity and our place in the world’.

HMS Queen Elizabeth is due to arrive in early 2017, while her sister ship HMS Prince of Wales is expected to be commissioned in 2020.