Pietersen has also urged Cook to relinquish the captaincy and seek emergency help for his batting - for the good of himself and England.
Cook has found himself under increasing pressure both as skipper and as opener following a miserable run which has seen England lose seven of their last nine Tests and the 29-year-old score just 129 runs in nine attempts.
The outspoken Pietersen, who was axed by England in February following the team's nightmare winter tour to Australia, feels Cook must now resign as captain and suggested it is only politics that are still keeping him in the role.
Writing in his column in the Daily Telegraph, Pietersen said: "At the moment only politics are keeping Cook in a job because the England and Wales Cricket Board backed him so much that it would be yet another PR disaster if it sacked him now.
"But the ECB needs his runs back more than anything else so a big decision has to be made. Forget the bad headlines for once."
The South Africa-born batsman does, though, believe Cook still has a lot to offer England if the left-hander can rediscover the form that has earned him a national record 25 Test centuries - two more than Pietersen.
"He should do what is right for England and resign the captaincy. He has shown he does not have the tactical brain to lead the side," said Pietersen.
"England badly miss Cook the opening batsman scoring 150 to set games up at the top of the order and, relieved of the captaincy, I don't think it is too late for him to rediscover this form.
"He could have another 10 years left but his batting requires emergency help and he needs to speak to someone away from the England set-up. He has to be honest with himself."
Pietersen feels Cook should seek advice from the many former internationals currently working as pundits, saying that is a valuable resource that is going to waste.
He said: "Cook needs people with experience of international cricket around him, which (England coaching team) Peter Moores and Paul Farbrace lack.
"Look at Marvan Atapattu working with Sri Lanka, Rahul Dravid with India, Shane Warne with Australia. How many ex-international players have England had recently working with them? None. There is so much knowledge in English cricket going to waste.
"There are so many great cricket brains in the Sky studios. Put that radio in your ear, listen to them on the balcony or in the dressing room and Mike Atherton, Nasser Hussain, Warne or whoever could be saying something you didn't know and could implement in the game."
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