Friday 25 June at 18:01 : Jun.25 (GMM) Lewis Hamilton on Friday admitted McLaren might struggle to keep up with rapidly-progressing rivals this weekend in Valencia.

While the British team will only run the innovation in two weeks at Silverstone, Ferrari, Mercedes and Renault have got Red Bull-style 'blown diffuser' exhaust layouts up and running on the streets of the Spanish port city.

Hamilton is the championship leader and has won the past two races, but on Friday afternoon was just fifth fastest, behind and surrounded by the 'blown diffuser' teams.

"It was always going to be interesting to see how quickly the blown diffuser cars could get up to speed -- and they looked very competitive this afternoon, particularly over a single lap," said the 2008 world champion.

"And the grip they appear to have through the high speed stuff could make them difficult for us to touch," added Hamilton.

His teammate Jenson Button, just ninth quickest in the final session, commented: "We've just got to fight it out here and get the best out of what we've got."