Mercedes non executive chairman Niki Lauda admits his squad are concerned that their competition are edging closer.
The Brackley-based squad dominated during the early stages of the 2014 campaign, but their rivals have managed to chip away at the gap in recent races, and Lauda admits they need to step it up again.
"The alarm bell is ringing that the competition is coming closer," he is quoted as saying by Autosport.
"But if the alarm bell rings in time you can do something.
"If you sit back, which we don't try to do, you miss out. So what we have to do is keep developing the car."