Alastair Cook was 'drained' by England captaincy - Andrew Strauss
Alastair Cook had become "drained" as England Test captain, says England's director of cricket Andrew Strauss.
Cook stepped down on Monday after a record 59 matches in charge.
"He was getting drained by the relentlessness of being England captain," Strauss told the BBC's sports editor Dan Roan.
Strauss added that vice-captain Joe Root would be a strong candidate to take over but refused "to rule anyone in or out of the role".
Cook is England's highest run-scorer in Test cricket with 11,057, while his 140 Test appearances and 30 centuries are also national records.
But the Essex batsman had been considering his future as captain after his side suffered a 4-0 Test series defeat in India last year.
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And Strauss said the 32-year-old had taken time to come to his decision.
"We know it has been a tough winter and it was an obvious time for him to step back and reflect and consider and have thoughts about what was right for the team moving forwards," he said.
"In my conversations with him in January it became clear that Alastair felt a huge amount of energy, drive and determination was needed to drive the team forward over the next 12 months.
"You are the only one who knows how much gas you have left in the tank and how much the many demands of being England captain are taking out of you.
"He feels it is time for new blood, new impetus and fresh thinking and allow someone else to take over and do that."
Strauss said he did not attempt to make Cook change his mind, and explained: "Once it became obvious how clear his thinking was, it was his decision to make. It would have been wrong to persuade him otherwise."
Is the appointment of Root a foregone conclusion?
The Yorkshire batsman, who was appointed England vice-captain before the 2015 Ashes Series is seen as the favourite for the job.
But Strauss, while praising his qualities, says that there is a process to go through before Cook's successor is announced.
England's next Test series will be against South Africa with the first game of the four-match series due to start at Lord's on 6 July.
After that, they will host the West Indies in three Tests in August and September before travelling to Australia for the Ashes in November.
"Joe has leadership experience and is a phenomenal cricketer and an influential figure in the dressing room, and there is no reason why he wouldn't be a strong candidate," said Strauss.
"But I don't want to rule anyone out or in at this stage.
"There are conversations that need to take place, both between myself and the selectors and the coach, but also among some of the senior players to make sure I understand how best to take the team forward so that when we announce the captain he is the right man for the job."
Pietersen decision 'won't define' Cook
Cook's first job after taking over from Strauss in 2012 was to manage the return of batsman Kevin Pietersen, who had been left out of the England side over allegations he had sent derogatory text messages about Strauss to members of the South Africa team.
But Cook also played an influential role in the decision to end Pietersen's international career in February 2014 when he was part of a three-man panel who met the batsman to tell him of their decision.
When asked if that incident could overshadow Cook's legacy as captain, Strauss said: "I think the fact he was able to get through that episode at a very tough time for him and others and come out the other side and keep scoring runs and winning matches and keep a degree of sanity at a difficult time speaks volumes for him."
BBC cricket correspondent Jonathan Agnew
The most difficult time for Cook as England captain was in 2014, which began with the Ashes whitewash down under, moved on to the Kevin Pietersen saga and was followed by a home series defeat by Sri Lanka.
His 2013 Ashes win as skipper is a highlight of his reign. So too, the triumph in South Africa in 2015-16 and the historic win in India in 2012.
Cook's winning percentage of 40.67 is only the fourth best of the six captains to have led England in more than 40 Tests. It has been an up-and-down ride.
The extended period of time taken to mull over his future shows that Cook has made the right decision for him. He will be incredibly comfortable with what lies ahead. That is likely to be scoring many more runs for England.