The regulator said insurers should work on a case-by-case basis on government support deductions, otherwise it could step in.
Poland softens its stance, while German transactions into London will end unless the UK is deemed equivalent, the Corporation warns.
Director general Huw Evans calls for a "relationship reset" between the watchdog and insurers after the court hearing.
Euler Hermes, Coface and Atradius are also participating in the scheme, which has had formal approval from the European Commission.