"Where are we? Quite a long way behind," 26-year-old Alonso said in Barcelona this week for the penultimate test of the winter.
"We are not in a position to do anything about McLaren, Ferrari or BMW.
"Ferrari are favourites but they're not in a different world. We however belong more to the group including Williams and Red Bull," he added.
Interestingly, despite the mixed messages coming out of the BMW Sauber camp recently, Alonso singles out the German team's 2008 car as one of the standouts of the pre-season.
"They are very fast," he said. "They always use full tanks (of fuel) and record very good times."