England's director of cricket Andrew Strauss insists Joe Root replacing "drained" Alastair Cook as Test captain is not a done deal but admits the 26-year-old is a leading candidate for the role.

Cook announced his resignation on Monday following four and a half years as skipper, and Root is now the outstanding favourite to take charge.

Root's leadership experience is limited, but he has served as vice-captain under Cook as well as becoming England's most reliable batsman since making his Test debut against India in 2012.

Ben Stokes, the talented all-rounder, is another potential contender, along with the more experienced options of Stuart Broad or James Anderson, but it would be a huge surprise if any of them were preferred to Root.

Strauss, however, says the Yorkshireman is not an automatic pick.

"Is it a given? I don't think so," Strauss said. "It's right we follow a process with this.

"There are a number of players playing informal and formal leadership positions, and it's right that we have conversations with all those guys."

One of Strauss' first moves upon taking up his role as England's director of cricket was, however, to appoint Root as Cook's understudy, and he has been impressed with his approach.

"It's worth saying I think he's done a very good job as vice-captain," Strauss said.

"He's obviously matured a lot over the last couple of years or so and there's absolutely no reason why he won't be one of the strong candidates for the role."

After 53 Test matches, Root averages 52.80, having scored 11 centuries and 27 fifties.

One concern is the captaincy could hamper Root's brilliant form, while another is that he will have less than a year in the job before undertaking the most formidable task in English cricket - leading an Ashes series in Australia.

Strauss, however, suggested neither would seriously harm Root's chances.

"Any England captain, no matter how good they are with the bat or otherwise, there are challenges to the role," the 39-year-old said.

"If you're one of the better players, it gives you more chance not to have to worry about your own performance.

"One of the unique challenges of being England captain is you have to perform at the same time as doing all the other jobs that this role entails.

"No-one is ever prepared for it until you do it. But a lot of people can grow very quickly when they're given this responsibility, and I think we've got a few people in this team who are already doing a really good job behind the scenes, who may well be ready to step up to that role."

The expectation is that Root would offer a more aggressive style of captaincy than Cook, who led from the front with his relentless run-scoring but had a tendency to err on the side of caution when it came to tactics and decision-making.

"More than anything, we just want clarity of thinking about how to take the team forward," Strauss said.

"You've got to drive people forward and you've got to have your ideas and your method.

"If a captain embeds himself properly, the team becomes a reflection of him and his personality."

Cook expressed doubt about his own drive to continue leading England in a meeting with Strauss after their 4-0 thrashing at the hands of India before Christmas.

The 32-year-old was urged to take more time to consider, but a period of reflection did not alter his thinking and Cook informed England and Wales Cricket Board chairman Colin Graves of his decision on Sunday evening.

"He was very honest with me from the start about some of the challenges about the role going forward and there are some significant challenges coming up and also how drained he was, it does take a lot out of you," Strauss said.

''He felt this was the right time for new energy for some new thinking and some new ideas and someone to grab it with both hands. It was the right thing for him and also the right thing for the England cricket team.''

Cook releases the captaincy after a national-record 59 Tests in charge, which included two Ashes series victories, as well as notable successes in both South Africa and India.

"His record stands for itself as England's longest-serving captain," Strauss said.

"To combine that longevity with his individual performances is a testament to his drive and character. I honestly believe he deserves to be looked upon as one of England's great captains."

Cook will continue playing for the Test side, with Sachin Tendulkar's record of 15,921 runs in sight. Cook currently has 11,057.

"I personally think that without the travails of the captaincy hanging over him, it will refresh him up immensely," Strauss added.

"It's worth mentioning he's only 32 years of age. He's potentially got three or four years left in him, at least."