Pressure is mounting on the UK government and opposition parties to commit to creating at least one massive marine reserve in the Pacific or Atlantic to protect rare and threatened whales, sharks, fish and corals ahead of the general election.

A coalition of over 100 conservation groups, prominent academics and film and TV celebrities have called on the Foreign Office to declare full protection zones around the Pitcairn Islands in the Pacific ocean, as well as Ascension Island and the South Sandwich Islands in the Atlantic. The three proposed reserves would together protect around 1.8m sq km (0.7m sq miles) of ocean and more than double the size of the world’s existing marine protected areas (MPA).

Declaring just one of the three proposed reserves ahead of the election would go some way to correcting the government’s perceived loss of environmental credibility, says the coalition which includes Greenpeace UK, the RSPB and the Blue Marine Foundation, as well as international groups including the National Geographical Society and the Pew Charitable Trusts.

Hugh Fearnley-Whittingstall, Gillian Anderson, Julie Christie and Helena Bonham-Carter are among the celebrities who have also signed a statement declaring that the creation of the reserves would make a “globally significant contribution to ocean conservation leaving a historic legacy for people and wildlife at very little cost.”

“The UK government has the opportunity to take global leadership in marine conservation, increasing the amount of the world’s ocean under full protection by 50%. These waters are amongst the most diverse on earth, with the British overseas territories housing 94% of the UK’s unique biodiversity,” said the coalition in a statement.

“If left unprotected these fragile ecosystems face huge threats from overfishing, illegal pirate fishing, pollution and climate change,” it said.

Britain was praised worldwide in 2009 when then foreign secretary David Miliband declared the world’s largest marine reserve around the Chagos archipelago in the Indian ocean days before the general election.

But this later proved contentious because the reserve appeared to preclude any resettlement of the atolls by islanders whose families had been evicted in 1965 to make way for a giant US air force base. This is now being negotiated.

Of the three reserves proposed by the coalition of conservationists, the least likely is one around the uninhabited South Sandwich islands because of its proximity to the Falklands. But in a statement the Foreign office said that it had started discussions with people living on Ascension island and Pitcairn.

“No decisions have been taken about designating marine protected areas around Pitcairn or Ascension but we are working with both territory governments and other stakeholders to determine the feasibility of establishing MPA in these territories,” it said.

“Government supports marine protected areas where they are scientifically justifiable and where an effective monitoring and enforcement regime can be established and funded,” it added.

It is understood that any MPA created would ban all commercial fishing around the inhabited islands but allow indigenous fishing to continue up to 18 miles offshore.

According to the conservationists, satellite monitoring and modern technology has now precluded the need for boats to patrol the vast areas and has dramatically reduced costs.

Previous government estimates, based on a single boat patrolling the Chagos islands MPA, have suggested it might cost £4m a year to monitor and enforce an MPA around Ascension or Pitcairn. But this may now have been reduced to £400,000 a year, said Charles Clover, chair of the Blue Foundation and a spokesman for the coalition.

“Enforcing and monitoring these marine areas would be cost effective. The Foreign Office is at a crossroads in dealing with overseas territories. It needs to recognise that we must deal with overfishing. We now have the technological ability to do this without boats and it is much cheaper. As it is, these areas are being plundered and are not being monitored at all, even though they contain 94% of all the UK’s biodiversity,” he said.

Pioneering satellite technology designed and developed in the UK by the Pew Charitable Trusts, the Satellite Applications Catapult and the UK government would allow analysts to identify and monitor illegal fishing practices in marine reserves as well as alert them to vessels acting suspiciously, he said.

The UK is responsible for the fifth largest area of ocean in the world, measuring 6.8m sq km, over twice the size of India, and nearly 30 times the size of the UK.

“It’s amazing to think that the UK is responsible for the fifth largest area of ocean of any nation in the world. That’s a big responsibility but also a great opportunity. We have a duty to look after our seas. Healthy marine ecosystems under full protection will help create greater future productivity in the surrounding seas,” said Fearnley-Whittingstall.