* Mercedes sweep front row, team's third successive pole
* Vettel third on grid, Raikkonen fourth (Adds Massa, Gutierrez penalties, Alonso quote)
By Alan Baldwin
BARCELONA, May 11 (Reuters) - Germany's Nico Rosberg put in a lightning lap to secure Mercedes' third successive pole position on Saturday with team mate Lewis Hamilton sealing a front row lockout at the Spanish Formula One Grand Prix.
The pole was Rosberg's second in a row after Hamilton, the 2008 world champion, took pole in China last month.
Red Bull's triple world champion and series leader Sebastian Vettel qualified third with Finland's Kimi Raikkonen, currently second overall and 10 points behind the German, alongside on the second row for Lotus.
Ferrari's Fernando Alonso, the local favourite at his home race, will start fifth.
"I had a really good lap in the end...it's always a goodmotivation, especially for the team. Front row. Fantastic. But of course we have to be a bit cautious because of what we saw in Bahrain," said Rosberg, who finished ninth in that race after suffering heavy tyre degradation.
"So it's nice, but to be enjoyed with caution."
Mercedes last swept the front row in China last season, a race won by Rosberg while then team mate Michael Schumacher failed to finish.
Saturday was the first time since 1955, when Argentina's Juan Manuel Fangio and Britain's Stirling Moss were racing for the 'Silver Arrows', that a Mercedes works team had chalked up three poles in a row.
The German manufacturer left Formula One as a works team in 1955, however, and only returned in 2010 after buying the title-winning Brawn GP.
"Am I confident for tomorrow? Not entirely," said team principal Ross Brawn. "I could see from fuel work that we're not on top of it as much as we are in qualifying but we've done a lot of work on that. Can we win? I don't know."
The statistics suggest Mercedes have a strong chance, given that only once in the last 22 years has a driver won in Barcelona without starting on the front row - and that was Schumacher in 1996 from third.
Alonso would have had team mate Felipe Massa alongside on the third row but the Brazilian was demoted three places for impeding Red Bull's Mark Webber, who moved up to seventh.
"Fifth place is more or less what we were expecting. We were well aware that Red Bull and Mercedes would both be very strong, but while it's true we have to start taking back points on Vettel, we are not too scared of Mercedes," said Alonso.
Sauber's Mexican rookie Esteban Gutierrez was also hit with a three place sanction, his second penalty of the season, for holding up Raikkonen and dropped from 16th place.
Hamilton, denied pole in Spain with McLaren last year due to a fuel irregularity, was fastest in the first two phases of qualifying but Rosberg made sure of the pole with a time of one minute 20.718 seconds.
Hamilton's best was 1.20.972, while Vettel's was 1:21.054.
"He did a great job...but as Nico said we are going to approach it with caution because tomorrow it's going to be tough," said the Briton, third in the championship.
"I didn't have two option sets (of tyres) at the end which would have helped a little bit but I think Nico was just quicker today."
There was despondency at Hamilton's former team McLaren, struggling to extract performance from their car, with 2009 world champion Jenson Button failing to make the final stage of qualifying for the first time this year.
The Briton will start 14th, with the new upgrade package brought to this race failing to live up to expectations more than anyone could have feared.
The only glimmer of hope for McLaren was provided by Mexican Sergio Perez, who qualified ninth.
"For Jenson, it's a disappointing outcome," said principal Martin Whitmarsh. "We've not analysed the data yet but there is some evidence that his tyre pressures were too high otherwise we hope he would have snuck through."
Further back, former champions Williams - last year's race winners with Venezuelan Pastor Maldonado - were in even deeper trouble.
A year on from the only win of his F1 career, Maldonado qualified 18th with Finnish rookie team mate Valtteri Bottas 17th.
"We've got to improve," said deputy principal Claire Williams. "You never panic in these situations. The grid is so tight, we were only one tenth off getting into Q2 so I don't think it's time to panic."
(Editing by Ed Osmond and Stephen Wood)