Williams warns against overlooking McLaren-Honda in 2015

McLaren completed substantially less mileage than its F1 rivals in testing amid repeated problems with Honda's new engine, and was also disrupted by an accident that left Fernando Alonso in hospital with concussion.

But asked by AUTOSPORT if he was surprised by McLaren's struggles, Symonds said the turnaround Red Bull and Renault achieved last year after a fraught winter showed what could still happen despite a bad start.

"If you rewind and think about some of the problems Renault had this time last year, they came through and won three races, didn't they?" he said.

"I think that was a fantastic tribute to what they did and therefore we shouldn't underestimate what anyone else can do."

Although Symonds believes Ferrari is currently Williams's main rival in the battle behind Mercedes, he did not rule out McLaren joining that fight.

"At the moment it looks like Ferrari [closest to Williams]," he said.

"They look to be the ones who have made the most improvement but you can't ever discount any good team.

"Red Bull will be there and once McLaren get through their problems I'm sure they will be.

"I try and spend more time looking forwards than backwards but you'll always have someone snapping away behind you."

Asked if he feared Williams would fall back towards the chasing pack rather than gaining on Mercedes this year, Symonds replied: "I don't think we're going to get sucked back. Whether they're going to be sucked into our tail, I don't know, it's hard to say.

"Winter testing is a bit of a false dawn. Year after year we try and see what the pecking order is and we do so often get it wrong.

"While one can't really quantify the gaps it sort of looks like Mercedes are still a little bit away from the rest of us, so the gap is still there.

"Behind that, ourselves, Ferrari, Red Bull, it's pretty close I think."