Britain's defences are at risk amid uncertainty over plans to replace the "woefully low" number of Royal Navy warships, MPs have warned.
The Royal Navy has 19 frigates and destroyers, but a Defence Select Committee report says that number could fall unless there is a clear timetable set out for replacing older vessels.
It says the UK could "lack the maritime strength" to meet potential threats.
But the Ministry of Defence says it is investing billions in the Navy's fleet.
The committee's report examines the MoD's plans to modernise the Royal Navy's escort fleet - including the introduction of two new classes of frigate and the enforced refit of engines on certain destroyers.
MPs said they had "serious concerns" about the funding and timetable of the new fleet, and the country's ability to handle threats from areas like Russia.
They also attacked the MoD for the "extraordinary mistakes" in the design of Type 45 destroyers after it emerged they had faulty engines unable to operate continuously in warm waters.
"The UK's enduring presence in the Gulf should have made it a key requirement for the engines. The fact that it was not was an inexcusable failing and one which must not be repeated," the MPs' report said.
And it added: "Failure to guarantee this would put the personnel and ships of the Royal Navy in danger, with potentially dangerous consequences."
Modernising the Royal Navy
- Ageing Type 23 frigates to be replaced by eight Type 26 frigates (pictured above) plus five General Purpose frigates
- First Type 23 ship due to retire from service in 2023
- Defence Select Committee says it is not convinced MoD can deliver to this schedule
- MPs also want detailed costs and timetable for refit of faulty engines on Type 45 destroyers
Defence committee chairman Julian Lewis said MPs were "putting the MoD on notice" to deliver the modernisation programme on time.
He said: "For decades, the numbers of Royal Navy escort vessels have been severely in decline.
"The fleet is now way below the critical mass required for the many tasks which could confront it, if the international scene continues to deteriorate."
The MoD says it is investing billions of pounds in two new aircraft carriers as well as new warships and submarines with the goal of increasing the size of the Royal Navy.
"This major programme of investment will ensure that the Royal Navy remains one of the world's most modern and powerful navies with a genuine global reach," it said in a statement.
It added that the Type 45 destroyer was a "hugely capable" ship, but it was "committed to improving" the vessel's "power and propulsion system".