Group Lotus-backed Renault driver Nick Heidfeld has suggested defending constructors' champions Red Bull have been sandbagging the new RB7 in pre-season testing.

Given Red Bull's Australian Mark Webber topped the time sheets on day one for the final pre-season test in Barcelona, the fact Red Bull could be even quicker is bad news for the rest of the field.

Heidfeld said: 'I think they are still not showing everything. If you look at the sector times they still have something in their pocket. So from those at least, there might be even more that they are not showing.

'I still think they are leading the pack at the moment. By how much? That is the question.'

Jenson Button declared himself much happier with McLaren's new MP4-26 after a productive day, while Hingham-based Team Lotus handed over the new T128 to rookie drivers Davide Valsecchi and Luiz Razia – both are racing for Lotus' feeder GP2 outfit, Team Air Asia this term.

Heikki Kovalainen and Jarno Trulli will return for the remaining three days of the Barcelona test.

Chief technical officer Mike Gascoyne said: 'We managed to run through our whole programme with the two drivers and they both did an excellent job…I want to thank both drivers for the work they have done in the car today.'

Virgin Racing were running the second of their new cars, which was only completed on Friday.

• Yesterday also saw the FIA promise to review all their current F1 circuits in a bid to increase overtaking – something set to become a hot topic over the coming season.

• Barcelona, fourth pre-season test – day one fastest times (all unofficial): 1 Mark Webber (Red Bull) 1 min 22.544 sec, 2 Jenson Button (McLaren) 1:22.910, 3 Vitaly Petrov (Renault) 1:22.937, 4 Sergio Perez (Sauber) 1:24.117, 5 Nick Heidfeld (Renault) 1:24.735, 6 Paul di Resta (Force India) 1:25.039, 7 Davide Valsecchi (Team Lotu) 1:25.406, 8 Sebastien Buemi (Toro Rosso) 1:26.004, 9 Nico Hulkenberg (Force India) 1:26.030, 10 Luiz Razia (Team Lotus) 1:26.030, 11 Jerome D'Ambrosio (Virgin) 1:32.060.