Uncertainty over plans to replace the "woefully low" number of warships risks jeopardising Britain's defences, an influential committee has warned.
MPs said they had "serious concerns" about the funding and timetable of the new fleet replacing frigates that are due to be decommissioned.
Dropping below the current 19 ships even for a short time would be "completely unacceptable" and leave the UK lacking the maritime strength to deal with the threats it faces from areas like Russia, the Defence Select Committee said.
MPs also attacked the Ministry of Defence (MoD) for the "extraordinary mistakes" in the design of Type 45 destroyers after it emerged they had faulty engines.
They accused the MoD and contractors of a "serious failing" for under-testing the system, which is unable to operate continuously in warm waters, and warned the problems had "potentially dangerous" consequences.
"It is astonishing that the specification for the Type 45 did not include the requirement for the ships to operate at full capacity - and for sustained periods - in hot regions such as the Gulf," the committee said.
"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," it added. "Failure to guarantee this would put the personnel and ships of the Royal Navy in danger, with potentially dangerous consequences."
Two new classes of frigate, the Type 26 global combat ship and the Type 31 general purpose frigate, are planned to modernise the fleet.
Some 13 ageing frigates are due to leave service at the rate of one a year between 2023 and 2035 but MPs said the Government has not set out the necessary detail on how and when they will be delivered.
"As an island nation, the importance of the Royal Navy to UK defence must not be underestimated," the committee said. "Our starting point in this report is our conviction that the current number of frigates, destroyers and personnel inadequately reflects the potential threats and vulnerabilities facing the UK and its interests overseas."
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.
"What remains of our surface fleet now faces a prolonged period of uncertainty, as the frigate class is replaced in its entirety and all our destroyers undergo urgent, major remedial work on their unreliable engines.
"The national shipbuilding strategy offers the potential not just to manage this work efficiently and effectively, but also to reverse the trend of ever-decreasing numbers. To do this, however, it has to contain the degree of detail and scheduling for which we have asked.
"The Ministry of Defence must deliver this programme of modernisation on time. If it fails to do so, the Government will break its categorical pledge to maintain at least 19 frigates and destroyers - already a pathetically low total.
"The United Kingdom will then lack the maritime strength to deal with the threats we face right now, let alone in the future. We are putting the MoD on notice that it must not let this happen."
An MoD spokesman said: "We are investing in a growing Royal Navy by building two aircraft carriers, the new Type 26 Global Combat Ship, Dreadnought and Astute class submarines, and offshore patrols vessels.
"We are also developing new class of Lighter General Purpose Frigate so that by the 2030s we can grow the size of the fleet. 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."
He added: "Type 45 Destroyers are hugely capable ships and have been deployed successfully on a range of operations worldwide. They continue to make an enormous contribution to the defence of the UK and our international partners.
"We're committed to improving the Type 45's power and propulsion system through a series of machinery upgrades during planned maintenance, and this work is progressing well."