The next phase of a massive multi-million pound plan to upgrade Portsmouth Naval Base has begun with discussions as part of an environmental assessment.

The plan, which could cost up to £200 million, includes the dredging of a deeper channel approaching the narrow harbour mouth – and may even require the dredging of a completely new channel.

The main channel in the harbour itself and the refurbishment of several jetties is also on the agenda, all of which would prepare the base for the arrival of the new Type 45 destroyers, due to enter service in 2008, and the two giant aircraft carriers which are expected to enter service in 2012.

Portsmouth has been recognised by the Ministry of Defence as being of environmental importance for a number of reasons, including nature conservation, fisheries and archaeological sites.

Accordingly, the Warship Support Agency has commissioned a firm of specialist consultants, Posford Haskoning, to undertake an environmental study in preparation for a more detailed environmental impact assessment later this year.

The WSA – part of the tri-Service Defence Logistics Organisation – has already started discussions with several organisations including English Nature and the environment Agency, and Posford Haskoning will be consulting with the statutory regulators and key stakeholders, including local authorities.

A much wider consultation exercise will be undertaken as part of the environmental impact assessment.

As part of the jetty refurbishment, due to begin next year, South Railway Jetty and Fountain Lake Jetty are due to be rebuilt, with others in line for possible upgrades.

New facilities will include aircraft carrier mooring off Stokes Bay, an upgrade of Naval Base facilities, including buildings, car-parking and roads, and improved access into the base’s largest basin, No 3.

Portsmouth Naval Base Commander, Commodore Amjad Hussain, said: “The start of this consultation process is an important and significant step in the plan for the regeneration of Portsmouth Naval Base, and demonstrates our commitment to minimising the effect of the work on both the historical and natural environment.

“The economy of Portsmouth can only benefit from the carriers base-porting decision, which ensures the continuation of Portsmouth Naval Base – the city’s largest direct and indirect employer – for the foreseeable future.”

The new aircraft carriers are the largest warships ever built for the Royal Navy and are some three times the size of the current Invincible-class carriers.

Two of these new ships are planned, weighing up to 70,000 tons each, with a beam of 74 metres at flight-deck level and 37 metres at the waterline. Their length will be just short of 300 metres, and they will have a draught of around 11 metres.