A prominent lawmaker in Greece's governing left-wing party has stepped down, hours before an important vote on reforms demanded by bailout creditors, saying he can no longer back government policies.

The resignation of Gabriel Sakellaridis, a former government spokesman, does not reduce Prime Minister Alexis Tsipras' parliamentary majority, as he will be replaced by another member of the Syriza party.

But his move points to increasing unrest in the governing coalition over unpopular reforms Tsipras has been forced to pass to secure the country's third bailout deal.

Tsipras was first elected in January pledging to reverse austerity measures. He was re-elected in September despite having broken most of those pledges to avert financial meltdown.

Proposed reforms to be voted on Thursday will include reducing struggling mortgage holders' protection from foreclosures.