Lotus will not run its new long-wheelbase car for the remainder of the Italian Grand Prix weekend.
Kimi Raikkonen tried the upgraded version in practice and was third-quickest, although he set an identical time to team-mate Romain Grosjean in the standard E21.
The team subsequently decided to play it safe for Monza, underlining that the decision was taken because of a desire to be conservative rather than any issue with the long-wheelbase car, which Lotus is convinced is at least as fast as its predecessor.
Lotus intends to bring the long-wheelbase package out again at a future race when there is more time to evaluate it.
Raikkonen said after practice that he had not felt much difference between the two cars.
"I think there is hardly anything in it," he said. "It feels very similar to other races."