The towering Serbia striker saw his campaign cut short by injury last season and could not help Blues avoid relegation to the Championship.
The Toffees have previously shown an interest in the 30-year-old, while his former club Racing Santander and Spanish rivals Malaga are also thought to be chasing his signature.
And while his agent, Milan Calasan, has refused to speculate over Zigic's next destination, he has revealed the forward will be leaving before the start of the new season.
"At the moment, the only thing that is sure is that Zigic is going to leave Birmingham this summer," Calasan told Serbian newspaper Sport.
"There is still a long time left to decide, so we are not in a rush. Nikola's wish is to continue his career in England or Spain."
The Birmingham squad will return for pre-season training on Tuesday, but former Valencia man Zigic is not expected to join them.
Calasan says the Serbian has already started individual preparation ahead of the new season to ensure he is fully fit after his injury struggles.
Zigic played a key role in Birmingham's successful Carling Cup campaign last term, scoring during their 2-1 victory over Arsenal in the final.
And his agent believes the Midlanders might have beaten the drop had the forward been fit for the final weeks of the campaign.
"Zigic has started the training process much earlier in order to fill the gap that occurred with his injury late in the season," added Casalan.
"Even more, I am sure that Birmingham might have avoided relegation if he played in those decisive matches at the end of the season."