Budget cuts to the Royal Marines and the number of amphibious assault ships they use would significantly undermine the UK's security, MPs have warned in a report.
The report comes amid speculation about the loss of 1,000 marines and a threat to the Royal Navy's two Albion-class amphibious assault ships.
The Defence Select Committee said "Treasury bookkeeping" could have a major impact on marine morale, already dented by recent cuts, as well as putting the country at "serious risk".
The Tory-led committee's report - called 'Sunset for the Royal Marines?' - will increase pressure on Theresa May to increase the defence budget.
"Given the disproportionate contribution the Royal Marines make to defence and the sheer range and versatility of their military skills, both they and the country's security would be significantly undermined," the report said.
"After more than three and a half centuries of service to the nation, Her Majesty's Corps of Royal Marines is in danger of being sacrificed to short-term Treasury bookkeeping."
Marine morale had already been hit and training curtailed, the MPs noted, adding it was a "particular embarrassment" that joint exercises with allies had been affected.
They also hit out at the reported threat to HMS Albion and HMS Bulwark, warning their capabilities could not be replaced by the new aircraft carriers or other warships.
"Disposing of our amphibious capability would not only put the interests of this country at serious risk, but would also be a drastic waste of tailor-made vessels, expensively refitted for another 15 years' use, and of a military specialism that has been fostered across all three services," they said.
The helicopter carrier HMS Ocean, another key part of the Royal Navy's amphibious capability, is already set to leave the service and is a "serious loss", the MPs said.
The modernising defence programme should ensure that enough money is available for both the carriers and amphibious capability, the Government was told.
Committee chairman Julian Lewis said: "In January, we were told that the Albion and Bulwark were not due to leave service until 2033 and 2034 respectively.
"That such irreplaceable ships are in line for deletion 15 years early demonstrates, yet again, the desperate inadequacy of the defence budget."