The EU Health Commissioner expressed "deep dissatisfaction" at the news.
Officials have not confirmed publicly how big the shortfall will be, but an unnamed EU official said that deliveries would be reduced to 31m - a cut of 60% - in the first quarter of this year.
The drug firm had been set to deliver about 80 million doses to the 27 nations by March.
The AstraZeneca vaccine, developed with Oxford University, has not yet been approved by the EU's drug regulator but is expected to get the green light at the end of this month, paving the way for jabs to be given.
A spokesman for AstraZeneca said on Friday that "initial volumes will be lower than originally anticipated" without giving further details.
His written statement blamed the discrepancy on "reduced yields at a manufacturing site within our European supply chain" and said the firm was continuing to ramp up production volumes.
Earlier, Pfizer Company stated to temporarily reduce the volume of vaccine supplies to EU countries during the period of increase of its production capacities.
The company plans to increase production from 1.3 billion doses to 2 billion doses per year during this period of time.