The car was undriveable - Button
By Sarah Holt
BBC Sport at Silverstone
British drivers Jenson Button and Lewis Hamilton struggled to make an impact at their home grand prix as McLaren were off the pace of title rivals Red Bull.
Hamilton lines up in fourth at Silverstone but was almost a second ahead of Button, who starts 14th.
"The car was undriveable," Button told the BBC Sport. "I think there's something wrong with the car.
Hamilton added: "Our car is very difficult to drive this weekend. It's very difficult to put a lap together."
McLaren had arrived at their home race at Silverstone with a major upgrade package which they hoped would help them close the gap on Red Bull's pace.
But after struggling with the balance and overheating parts during Friday practice, the team decided to abandon their version of the exhaust blown diffuser and only run with the new front wing for qualifying.
Hamilton and Button were only able to run the modified car for an hour in final practice but the data gathered from that session resulted in differing fortunes for the successive world champions.
"I am very disappointed," said Button, who starts in his lowest grid position since qualifying 17th at a rain-hit Malaysian Grand Prix.
"I wanted to have a good day here in front of the home crowd so it's frustrating to be so far back.
"This morning the car felt good. We didn't actually change anything going into qualifying because I was happy with the balance.
"But going into qualifying I had very low rear grip and it felt like I had less downforce. I don't know why as we haven't had a look at the car yet.
"It's very sad as I felt confident this morning and happy with the car. This weekend has been tricky but something felt wrong."
Jenson Button was an unhappy man after qualifying for the British GP
But Hamilton, who won the British Grand Prix in 2008, managed to squeeze more pace out of the car and finished fourth behind Webber, Sebastian Vettel and Ferrari's Fernando Alonso.
He said: "I had to nail that lap as if my life depended on it - and I don't know how I managed to get so far up the grid.
"I literally got everything out of the car, even though we're not on pole I felt we optimised it. Fourth position makes me happy like you wouldn't believe!
"The new parts are an improvement, but we just weren't able to get them all to work properly yesterday. But while that meant we had to take most of them off last night, the team still did a remarkable job to get us ready for qualifying."
But after taking pole and the victory in Canada and leading home a McLaren two-three at the last race in Valencia, Hamilton is disappointed to be trailing the Red Bulls as he aims to defend his lead in the world championship on home soil.
"In terms of pure pace it's impossible to touch them until we get the blown diffuser," added Hamilton.
"Our car is amazing through the high-speed corners but the Red Bulls are flat out through Turn One and I can't get flat out through there.
"I'm just going to follow them as closely as possible. Their reliability isn't as good as ours so hopefully we can catch them."
In an attempt to boost the McLaren's competitiveness, test driver Gary Paffett returned to the McLaren factory on Friday evening to drive the team's simulator, which was set up to replicate the car without the new diffuser.
Both Button and Hamilton had hoped to run with the aerodynamic device, which can provide a lap-time gain of about 0.5 seconds, but Hamilton praised the team's efforts to at least give them a fighting chance in Sunday's race.
"The team did everything they could to get the update package ready," commented Hamilton. "I wasn't keen to revert back.
Hamilton has made the top four in qualifying for the last four grands prix
"But we had some problems and Gary went back to the simulator. We analysed the data and used it today - some of it worked out better."
Button added: "It was the right decision to take the upgrades off for now, there is a lot of work we still need to do on them, so it was the right decision."
McLaren team principal Martin Whitmarsh says he took a gamble introducing the updated diffuser at Silverstone but is confident it will provide a useful tool in the chase to match Red Bull's blistering pace.
"It was planned for Germany and we tried to bring it forward," added Whitmarsh. "We brought it here a little bit earlier than we should have. It was a risk but we took it.
"It's a disappointment that we're not quicker than the Red Bulls here but I'm sure we'll be doing some things in [the next race in] Germany.
"Lewis holds the view that perhaps if we'd have kept the blown diffuser on we'd have been even quicker but we made a judgement call.
"With the variability - it was gusty, it was bumpy, it's a new circuit and we didn't get it right - so we had to eliminate some variables and that's what we chose to do.
"We wouldn't be putting all the effort in if we didn't think it had that sort of potential of maybe half a second."