The bluetongue protection zone in Wales is unlikely to be rolled out before 17 August, according to the Welsh assembly.
The latest indications of vaccine production signal that batches will be available later than expected, the assembly said.
This means the roll-out will occur after the surveillance zone in England has become a protection zone.
An assembly statement reassured worried sheep farmers that the situation would be reassessed should there be any development in the disease situation, or a change in the vaccine delivery timetable.
“The situation with bluetongue can change very quickly,” said chief veterinary officer Dr Christianne Glossop.
“We are sharing this news of delay with the industry to help farmers make plans.
“I hope this information will help farmers in south west Wales in particular.”
Mary James, NFU Cymru deputy director, said the news was a setback and would disappoint farmers anxious to get the security that vaccination would bring.
Meanwhile DEFRA has announced the bluetongue protection zone in England will be extended on Monday (21 July) following delivery of 2m additional vaccine doses.
The protection zone will cover Worcestershire and North Yorkshire, as well as Cheshire and Greater Manchester.
DEFRA also confirmed the Merial BTV-8 vaccine has been awarded a provisional marketing authorisation from the Veterinary Medicines Directorate.
This was an important step in confirming that the vaccine will be safe and effective, it said.