Bernie Ecclestone estimates Germany's chances of staying on the 2015 calendar at "less than 50 per cent".
The Nurburgring was due to return to the schedule in 2015, but German reports say the boss of new circuit owner Capricorn's offices were raided last week by local prosecutors amid allegations of fraud.
And F1's other German host, Hockenheim, is yet to agree contract terms with Ecclestone about stepping in at short notice.
"The chance that there will be a German Grand Prix this year is less than 50 per cent," F1 chief executive Ecclestone told Auto Motor und Sport late on Thursday.
"If we come to the middle of February or the beginning of March and still have not agreed, it will be difficult," the Briton added.