Spanish Grand Prix
Venue: Barcelona Date: Sunday 9 May Start: 1300 BST Coverage: Live on BBC One, Red Button, Radio 5 live, online and mobiles
BBC coverage in detail
Button was ninth fastest, 0.173 seconds slower than his team-mate Lewis Hamilton
By Sarah Holt and Andrew Benson
BBC Sport at the Circuit de Catalunya
McLaren's Jenson Button says Red Bull are unbeatable at the Spanish Grand Prix after Sebastian Vettel and Mark Webber finished one-two in practice.
Vettel was 0.8 seconds quicker than the fastest non-Red Bull, which was fellow German Michael Schumacher's Mercedes.
"It's horrible to say it, but I think Red Bull are untouchable this weekend," said Button, the championship leader.
"We're not happy with how the car feels so we're not getting a good lap times, but the Red Bulls look very quick."
Vettel has been on pole position for three of the four races so far this season, with team-mate Webber the only other man to occupy the front of the grid.
The German was the only driver to dip below one minute 20 seconds at the demanding Circuit de Catalunya as he clocked 1:19.965 in the second session.
Vettel's time was 1.226secs quicker than the fastest McLaren lap, which was set by Button's team-mate Lewis Hamilton, who was fifth behind Ferrari's Fernando Alonso.
Button was ninth fastest, 0.173secs slower than his team-mate and behind the Renault of Robert Kubica, Nico Rosberg's Mercedes and Alonso's Ferrari team-mate Felipe Massa.
Button said: "To be in the 19s is a very quick lap time. Red Bull's low-fuel runs are, as usual, much quicker than their high-fuel runs but they are quick no matter what fuel they are running."
Teams run low fuel loads when they are preparing for qualifying and high-fuel loads to prepare for the race, which they have to start with full tanks of about 210 litres of fuel.
Alonso, the man who has been closest to Red Bull's pace in qualifying this season, said Ferrari had changed their approach to Friday practice in an attempt to better adapt their car for qualifying but admitted that even he would struggle to match the Red Bulls.
"Red Bull were very, very quick in second practice and it will be very difficult for all of us to arrive to those times," the Spaniard said.
"We see some times some good Fridays for some people and then on Saturday they struggle a little bit more.
"So maybe we can find something to be close to them and then hopefully they can do some mistake in the set-up or something and we can close the gap.
"But in normal conditions it is going to be very difficult to beat them this weekend."
There were early predictions that the weekend could play into Button's hands as the demanding combination of long corners in Barcelona requires careful tyre management, an area in which he excels.
But Button - who won this race for the Brawn team, now renamed Mercedes, last year - said he had struggled during Friday practice.
"We've still got a bit of work to do," he admitted. "I didn't get as much running as I would have liked in the morning as I hit something coming out of the pits and damaged the barge board.
"We made some set-up changes in the afternoon and thought it was in the right direction but I'm not sure if it was.
"There is a lot of work we need to do on both tyres; some of it is possibly the set-up and some of it is getting the tyres in their working range.
"It's been a tricky session but I don't think it's an issue of the new parts not working, I just think we're not turning the tyres on correctly or working them in the right way."
In common with all the teams, McLaren have brought a major upgrade package for their cars to the Spanish Grand Prix.
Hamilton, who finished second for his career-best finish in Barcelona in 2007, added: "It's surprising to see the gap between us and the Red Bulls.
"We've made a step forward but it'll be interesting to see tomorrow how big a step it is compared to the others.
"It does seem that one team is very quick at the moment. We need to keep pushing and working as hard as we can to close that gap."