The Commons public accounts committee warned that the HLF expects its income to fall by £57 million to around £180 million in 2009-10.
However, it said uncertainty remains over the impact of Lottery scratch cards with income ring-fenced for the Olympics.
Its report states: "The Department for Culture, Media and Sport (DCMS) should provide the HLF with a frank assessment of when and by how much its income will fall and its goals and expectations for recouping money from the sale of land."
A DCMS spokeswoman confirmed that land sales after the Olympics would recoup the games' cost, but admitted that nothing could be guaranteed because future land values are unknown.
Grants already awarded by the fund are secure, but an HLF spokeswoman said cuts would result in much tougher competition for applicants. The report's recommendations will now be considered in detail, she added.
English Heritage welcomed the government's announcement last week of an increase in heritage funding of £7 million in cash terms over the next three years following the comprehensive spending review.
The heritage body's chairman Lord Bruce-Lockhart said: "This increase in resources will allow us to move forward on a number of key heritage initiatives."