A funding shortfall means the World War One warship HMS Caroline will stay closed to public until next year.
The National Museum of the Royal Navy (NMRN) said the Covid-19 closure had left a shortfall of £6.35 million.
It claimed a funding agreement could not be reached with the Department for the Economy (DfE).
Caroline is the only surviving ship from the Battle of Jutland and is berthed in Belfast.
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The department said it had "deployed external consultants" to examine the deficits.
UUP leader Steve Aiken described the decision to delay reopening as "absolutely scandalous".
NMRN, which operates the attraction on behalf of the executive, said it is "under extraordinary financial pressure".
It said it was told an emergency grant would be made available, enabling NMRN to reopen to the public.
However, the funding model for HMS Caroline is different.
"Whilst the ship is owned by NMRN, it is opened to the public through an operator agreement made with the Department for the Economy in Northern Ireland," the NMRN said.
"NMRN operates the ship seeking to maximise income to cover costs, but if there is any shortfall then the DfE guarantees to pay the excess.
"This agreement has been in place since the ship reopened the ship in 2016 until 30 June 2020 when it expired, and means the responsibility for the operation of the ship has now fallen back to DfE."
The statement said it had "worked extensively" to set up a new operational agreement.
NMRN said this had not been possible and claimed the DfE has "therefore chosen to mothball the ship for six months whilst they decide what to do with it".
Dominic Tweddle from the NMRN said they would "continue to fight to secure the future of the ship and its team".
"However, if we are not able to sway DfE from its current position then HMS Caroline will not reopen until 2021 and those jobs will have to be made redundant."
A DfE spokeswoman said visitor numbers had been "disappointing to date resulting in operational deficits".
"The department has been concerned about these deficits for some time and has been working closely with NMRN, who also act as operators, to attempt to make the attraction more profitable, reduce deficits and ensure greater value for the use of public funds," she said.
"However, due to the Covid-19 crisis the attraction closed to the public on 17th March 2020.
"The department was first informed of operational deficits in October 2018 and NMRN has still been unable to verify the totality of these."
She said the department had "deployed external consultants to fully establish and confirm the totality of this deficit and this work is still ongoing" and it had "made a substantial interim payment" to NMRN to help with cash-flow issues.
She said the agreement between the department and NMRN expired on June 30.
But NMRN "took the decision not to renew this agreement without a revised funding model being put in place" on June 10, which did "not leave sufficient time" for a new agreement to be made.
The department said it agreed with NMRN to extend the current period of closure to December 31 and will use the period to examine all options.
