Williams performance chief Rob Smedley believes it will be very hard to catch rival Ferrari given the Italian squad's current form.
The Grove team started the year hoping to be Mercedes' closest challenger after a strong 2014, but Ferrari's turnaround has meant Williams has found itself with the third best car.
As Ferrari continues to ramp up its efforts to catch Mercedes, Smedley reckons the Maranello squad could be out of reach.
"After the first four races you can already see the balance of power," Smedley said after Valtteri Bottas finished fourth in the Bahrain Grand Prix.
"We are preparing a series of upgrades for Barcelona, but catching up with Ferrari will be very difficult.
"We have already extracted good potential from our cars and this work continues, but we do not expect to be able to rapidly increase our pace."
Smedley is also wary of the threat from Red Bull, which he expects to bounce back from a poor start to the season.
"We can't forget that behind us there are teams like Red Bull, which is a huge organisation, well equipped and with a very considerable resource," he added.
"When they sort out their problems, they will be doing much better than they are doing now. Generally, all of our main rivals are very strong."