The Gala player damaged his knee playing against Hawick last weekend but the full extent of the damage was not discovered until Saturday, when Townsend was examined by the Scottish Rugby Union doctors.
"Gregor has damaged the posterior cruciate ligament in his left knee and could be out of the game for up to a year," the Scotland manager, Duncan Paterson, said.
Townsend will undergo a further examination in Edinburgh today to determine whether surgery is necessary. If it is not, then Townsend could be back for the start of next season but surgery would mean an absence of a year.
Paterson said he was optimistic about the chances of the Bath duo, Andy Nicol and Andy Reed, being fit for the World Cup. Both have been out of action for most of the season but Nicol has already played two games for Bath reserves. The Bristol prop Alan Sharp is still suffering from a back problem and has only a even chance of going to South Africa.