"Multiple technical failures" at Ocado's Hatfield depot have resulted in late deliveries and large numbers of missing goods for customers over the past two weeks.
The problem has been blamed on the company's automated systems.
"Ocado's ordering and picking software is not working," said a source close to the company. "It has caused massive problems over the past few weeks and is still not fixed."
Depot workers were having to complete ordering and picking manually, which was costing a significant amount in extra manpower, he added.
One customer who regularly shops with Ocado, said six fresh produce items on his list were not delivered for two consecutive weeks.
Ocado declined to comment. A letter of apology to shoppers from head of customer service Mark Bentley read: "These poor levels of service are extremely unusual." Affected customers have been issued with money-off vouchers.