The Ministry of Defence will confirm the £3.8bn contract for two new aircraft carriers after the Scottish elections, according to Whitehall sources.

An announcement has been delayed by hard bargaining between the UK yards sharing the work and Lord Drayson, Procurement Minister, and the need to avoid accusations of using the promise of jobs to influence voters ahead of the May 3 polls.

But the Royal Navy will then face the mothballing of two Type 42 destroyers and four Type 22 frigates to help save a £250m overrun in fuel, maintenance and other costs.

The cutback, which includes HMS Cornwall, the warship whose crew members were taken hostage by Iran's Revolutionary Guard forces, could also mean scrapping one or even two major global naval commitments.

Insiders say the tasks under threat are the Falklands guard ship, whose removal would leave the islands vulnerable for the first time since the 1982 war with Argentina, and possibly the Indian Ocean anti-drugs patrol.

Axing or reducing the South Atlantic cover would be particularly embarrassing at a time when the UK is preparing to celebrate the 25th anniversary of the recapture of the islands and Argentina has lodged new claims over their sovereignty.

The reductions would leave the Navy with just 19 destroyers and frigates until the six Type 45 air defence destroyers on order from BAe's Clyde yards enter service between 2009 and 2013.

The current 25-strong surface escort fleet has already reduced patrols in the Caribbean and North Atlantic because of shortage of numbers and cut its two-ship presence in the Indian Ocean by half.

The First Sea Lord, Admiral Jonathon Band, warned in February that he needed a further £1bn a year simply to keep the existing warships operational and to improve pay and living conditions for sailors under his command.

A senior naval source said yesterday: "The RN had 98 ships in service in 1997. A decade on,we have 75. Only 25 of those are escort vessels and many of the others are supply and support ships, inshore and offshore patrol craft and survey vessels.

"The carrier Invincible is effectively out of the picture at low readiness', although she remains on strength. It would take a minimum 18 months to make her seaworthy. Five other ships are at extended readiness'. It would take more than 180 days to make them operational."

A review is also under way to slash costs at the UK's three naval bases. It cost £183m to operate the Clyde submarine base at Faslane last year, £185m for Devonport and £151m for Portsmouth, which is seen as the most vulnerable.