Kimi Raikkonen believes Mercedes must up their game after a worrying start to testing for the new McLaren.
Raikkonen fears engine trouble could plague McLaren again
Raikkonen expressed his concern over McLaren's new Mercedes V8 engine, which has suffered reliability problems since it made its debut last week.
"They are not where they should be and they need to improve. On the engine side we have a lot of work to do," the McLaren driver told www.autosport.com.
"It is definitely our weakest link now, but improvement is coming."
McLaren saw their 2005 campaign dogged by reliability issues as the team was beaten to the driver and constructor world titles by Renault.
Regular wins last season were interspersed with problems that either forced Raikkonen into retirement or relegated him to grid positions from where it was impossible to beat rival Fernando Alonso.
Raikkonen now fears history could repeat itself this term, in which new 2.4-litre V8 engines have replaced the three-litre V10s that have been used in F1 since 1995.
"We have been testing for a long time but we have not been able to make progress," he said.
"It is the first time I am in the car. I know there is a lot of work to do but they are improving, and I know what is coming.
"I am slightly worried but not too much. Sure I can expect more but now we'll wait for a proper engine and we'll see."