With HRT already a non-member, the latest development follows last week's news that the biggest-budget Ferrari and Red Bull teams had pulled out.

International media reports quoted official spokespeople for Swiss based team Sauber as confirming that notice has been given to FOTA chairman Martin Whitmarsh.

"The reasons were communicated to the teams of the association, but they will not be made public," Germany's Sport1 reported, citing a team official.

Sauber, like Red Bull junior team Toro Rosso, is powered by Ferrari engines.

"Toro Rosso has not yet announced its departure, but in practice it is certain," wrote Finland's Turun Sanomat newspaper.

The report said Toro Rosso - like Red Bull Racing, Ferrari, Sauber and HRT - was not represented at Tuesday's emergency FOTA meeting in London.