Alastair Cook has been branded "mentally the toughest cricketer" in England's history by former captain Nasser Hussain after announcing his impending international retirement.

Cook is his country's all-time leading Test run scorer with 12,254 and will call time on his England career at the end of the series against India.

The 33-year-old has appeared in 160 Tests having debuted in Nagpur 12 years ago and he will have featured in 159 successive five-day games if he is selected to face India at the Oval.

Hussain hailed the ex-skipper's longevity and believes his run in the team is a testament to his character.

"He has been a truly remarkable cricketer," Hussain told Sky Sports News.

"To carry on this long … he's played 158 Test matches in a row - you'd think somewhere along the line you'd lose form or fitness, [he is] mentally the toughest cricketer I've ever known as far as England goes.

"Huge credit to the lad, he's been England's best player."

Cook led England in a record 59 Tests and was the captain for two Ashes series victories in 2013 and 2015.

He also scored a staggering 766 runs in the series victory in Australia in 2010-11, when spinner Graeme Swann was one of his colleagues.

"He is one of the nicest men alive," Swann said on BBC Radio 5 live.

"He has turned himself into a holder of all these batting records. He is a dear friend and I'm so glad this is the right time for him.

"Even if he was in bad nick there was always a tantalising thought that he would score a double hundred.

"There is a reason why he has lasted so long. He always put in 100 per cent but would look and appreciate the other side of life."

Cook has struggled for runs in the ongoing series with India – accumulating just 109 at an average of 29 – and ex-England wicketkeeper Alec Stewart feels he is making the right decision.

"I think he has timed it perfectly," Stewart added.

"He will still go out as England's greatest run scorer. We won't fully appreciate him until he has gone.

"He has broken all these records, he is the heartbeat of the team in such an unassuming way. There is some madness in opening the batting and he has done it so well."

Cook, a mainstay at the top of England's order for over a decade, has had numerous opening partners in recent years.

However, there will be a new combination and former seamer Bob Willis urged the selectors to choose Surrey's Rory Burns, who has scored almost 1,000 runs in the County Championship this season.

"He will get his opportunity," Willis told Sky Sports. "That's a problem area for England – one, two and three.

"The hardest job is at the top of the order against that new ball and that's going to be difficult for England to replace."