Eric Boullier says that Lotus has to 'push on' when the F1 season resumes in Barcelona to ensure that the team continues to fight with the best at the head of the pack.

The Enstone-based team has already tasted victory this year thanks to Kimi Raikkonen, who sits second in the championship standings behind defending champion Sebastian Vettel as a result.

Lotus are also second behind Red Bull in the constructors' championship following the opening four races of the campaign, with Romain Grosjean having taken his first podium of the year in Bahrain to boost the team's tally.

On the back of that solid start, Boullier said he felt the team was now firmly in contention at the front of the field but would need to work hard to maintain the momentum it currently has and to ensure it doesn't fall away from the title fight.

"It is always difficult to keep any momentum actually, but we can say we are fighting with the best now," Boullier was quoted by AFP. "The challenge the team faces going forward though is in ensuring that it has the development potential to push on."

Lotus is set to follow its rivals in introducing a new upgrade for the start of the European season in Spain next month, and Boullier said the improvements planned for the E21 were on track.

"We have an update for the car, just like everybody," he said. "I don't know what Red Bull means by massive - so we will see!

"But we will all bring a nice package and I promise you that the development of this car is going very well."