Don't write us off: Schumacher

Ferrari's Michael Schumacher has warned rivals not to write off his Formula One championship challenge despite the team's lack of success so far this year.

"Anyone who wants to write us off doesn't know us," the seven times world champion said on his personal website after he retired from Sunday's Spanish Grand Prix for his third non-finish in five races.

"Of course this result was anything but productive in regard to the fight for the championship title, there's no two ways about that.

"But the first few races this season showed how much can happen during just one weekend and how quickly everything can change to somebody else's favour in Formula One.

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"So I'll say it again: The season is very long and there is a lot of room for things to happen. Sure, the fight for the title has become a lot more difficult for us," added the 36-year-old German.

"But nothing is lost yet. We will carry on working and we will carry on fighting ... it's not over yet."

Championship leaders Renault won the first four of the season's 19 races with McLaren's Kimi Raikkonen triumphant in Spain. Schumacher has 10 points to Spaniard Fernando Alonso's 44.

Ferrari won 15 of last year's 18 races with Schumacher victorious in 13.