About 32,000 tickets are still unsold, with a record number of returns from tourism operators and travel firms last month.
Demand from south of the Border – which usually accounts for more than half the ticket sales – is said to be particularly slow.
However, organisers refused to blame the clash with the London Olympics and said the sales figures reflected a growing trend for late booking and wider economic factors.
Tattoo producer Brigadier David Allfrey said he was “very confident” the show would eventually sell out, although he said people would still be able to snap up tickets on the day of performances.
But he admitted “harsh trading conditions” were making it much harder to sell tickets than in previous years.