Britain's fleet of attack submarines could shrink from seven boats to six, after the next to enter service was reportedly delayed by 10 months.
The Royal Navy currently has four Trafalgar-class subs in service, along with three, newer Astute-class vessels. Another four will eventually replace the older boats.
According to a Ministry of Defence report seen by the Times, however, the fourth Astute boat, HMS Audacious, will now not enter service until November 2018, while Trafalgar-class HMS Torbay is due to be decommissioned this year.
Audacious was initially due to enter service in August 2015, but this had already been delayed until January 2018.
The report also revealed that the cost of building the four new subs rose by nearly £200 million last year, according to the newspaper.
They're used to monitor foreign submarines near British waters, as well as protecting the UK's warships and nuclear subs.
It comes after reports last week that all of the current attack submarines are out of action. Lord West, a former head of the Royal Navy, said:
"I think it is highly likely — and this is worrying — that we will drop below having seven [attack submarines]. I think we will drop to six. To keep two operational at sea, it is quite a push with seven, and you can only do it for certain periods with six."
Defence analyst Francis Tusa, meanwhile, said reducing the fleet to six meant "[the existing attack submarines] will have to work even harder, which means the crews will be spending more time at sea".
The MoD, for its part, said in a statement:
"The Royal Navy's attack submarines meet their operational tasking and will continue to do so over the next decade. It is not uncommon to have temporary small fluctuations in overall numbers during transition from one class to another."
BAE Systems, which is building the submarines, said designing and building them "is one of the world’s most complex engineering challenges".
The MoD report, meanwhile, also revealed that the cost of a number of large defence projects rose last fiscal year, compared to a fall in the previous 12 months.
There was an overall increase of £237m for the biggest projects against a fall of £247m in 2014-15.
Among the cost rises was £249m for building nuclear reactors for the Dreadnought subs that will carry the Trident nuclear deterrent when the Vanguard-class retire, and a plan to integrate Brimstone missiles onto the RAF's Typhoon jets.
This rose by £49m last year, although the MoD said costs had subsequently fallen.
The programme to upgrade the British Army's Warrior armoured vehicles increased by £40m, meanwhile, and was delayed by a further year to 20 months.