Bottas: It will be difficult to close the gap to Ferrari

Williams driver Valtteri Bottas has echoed the team' technical boss Rob Smedley in playing down his team's chances of chasing down Ferrari in 2015.

Team deputy Claire Williams admitted in Bahrain that the Grove team had hoped to build on its promise of last year and take on champions Mercedes in 2015.

"That's what we wanted to do this year," she said, "and it hasn't worked out like that."

But Williams insisted the famous British team will "keep plugging away" at the factory despite its smaller budget, culminating in significant upgrades for the FW37 to debut in Barcelona next month.

Chief engineer Smedley, however, warned that not only will Williams struggle to "rapidly increase our pace" and catch Mercedes and Ferrari, but that fending off a late surge from Red Bull may also prove difficult.

He said Red Bull is "a huge organisation, well equipped and with a very considerable resource".

Nonetheless, Finnish team driver Bottas said he is hopeful the Barcelona upgrades will enable Williams to keep pushing ahead.

"But this year it will be difficult to completely close the gap because Ferrari made a huge step forward since the first race," he said.

"I'm confident that we will keep improving, but of course it will then depend on how much the teams in front improve as to whether we can catch up with them or not," added Bottas.

The 25-year-old has returned to his home in Monaco this week, pleased that a back injury that sidelined him in Melbourne has now completely healed.

"Bahrain was the first weekend since Melbourne in which my back gave me no problems at all," said Bottas. "But I will still do a bit of work on it to complete what [rehabilitation] was planned."