Del Monte suffered a 30.5% drop in value sales of its fruit juices to just under £21m in the 52 weeks to 2 December, according to ACNielsen.
An industry insider said the decline can be attributed to the growth of own-label products, driven by fierce price promotions.
While this drop-off is affecting the entire ambient juice category, Del Monte has experienced the biggest decrease.
'The market has changed significantly,' said the insider. 'A couple of years ago, people primarily drank fruit juice with breakfast; now it is being drunk more by consumers on the move.'
Leigh Edwards, commercial director of branded groceries at Del Monte, said: 'Our brand has been in decline for a few years and some retailers have discontinued it in favour of new products.'
Edwards added that Del Monte is investing heavily to reverse its flagging fortunes, including a £3m promotional campaign it ran this autumn.
'We launched the Del Monte juice bar range in June, which has been hugely successful. We are also concentrating on reaching out to a younger audience by increasing our focus on the impulse category,' he said.
Del Monte is not the only ambient juice brand facing up to plunging sales; Ocean Spray experienced a 15.7% slump in value during the same period.
However, Princes has benefited directly from Del Monte's delisting, with its sales increasing 58.7%.
While sales of ambient jucies are falling, chilled juices are seeing significant growth as health-conscious consumers increasingly choose fresh chilled juices; sales of Tropicana Pure Premium have increased 34.2%.