Mont Orgueil Castle is one of the sites run by Heritage
Weak commercial management and falling visitor numbers have contributed to the "poor financial condition" of Jersey Heritage Trust, a report concluded.
The Comptroller and Auditor General, Chris Swinson, has reviewed the organisation following public concerns.
He found there had been an annual and growing deficit since 2005, and this year's deficit could reach £190,000.
The trust said the current level of funding is not sufficient to maintain the services it offers.
The organisation works to maintain, manage and promote key landmarks in Jersey such as Mont Orgueil Castle, Hamptonne and La Hougue Bie.
Earlier this year, it warned it may have to close two sites because of a lack of grants.
The trust's main source of funding comes from the States' annual grant.
Jon Carter from Jersey Heritage blamed the problems on falling tourism figures
In his report Mr Swinson concluded: "The financial problems confronting the trust are urgent.
"It is unlikely that the level of annual grant it has received in recent years will be sufficient to sustain the existing activities even if the overdraft is repaid.
Mr Swinson described the trust's approach to budgeting and cost control as "being consistent with an organisation that attempts to spend money that it expects to be available rather than one that attempts to ensure its costs are appropriate and controlled.
"By the end of 2009, the trust's overdraft is likely to amount to between £800,000 and £900,000."
Jon Carter, the director of Jersey Heritage, said: "The background to our troubles is the decline in tourism.
"The report says we need to take a much closer look at some of our costs which we're doing currently."
Deputy James Reed, the education, sport and culture minister, said: "In line with recommendations contained within the Comptroller and Auditor General's report, the department has been working closely with the Trust to secure its future.
"Auditors appointed by the department are working with the Trust to review governance arrangements and to agree a method of financial reporting and allocation of costs consistent with the approach advocated by the Comptroller and Auditor General."
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