|First Test: England v India|
|Venue: Trent Bridge Dates: 9-13 July|
|Coverage: Ball-by-ball Test Match Special commentary on BBC Radio 5 live sports extra, tablet and mobile devices. Live text commentary on the BBC Sport website/app|
Alastair Cook's England captaincy will be defined by the result of this summer's Test series against India, according to Michael Vaughan.
England, who have failed to win their past eight Tests, start the five-match series at Trent Bridge on Wednesday.
Former captain Vaughan believes Cook, 29, should remain in his role.
But he told BBC Radio 5 live: "The key is winning. That's what will define Alastair Cook over the next five Test matches. Can he beat India at home?"
Cook is England's fifth-highest Test run-scorer, with 8,125 runs, and has made 25 centuries in 104 matches since his debut in India in 2006.
But his form is under scrutiny after 24 Test innings without a century, while Australia legend Shane Warne is among those to have criticised his captaincy.
Speaking on his Tuffers and Vaughan Cricket Show, Vaughan added: "I don't feel it's the right time to change the England captain. Alastair Cook's the man. He's certainly the one who can change the way England are playing."
Cook replaced Andrew Strauss as Test captain after England's 2-0 defeat by South Africa in 2012 and led them to their first series victory in India for 27 years.
There were also home series wins over New Zealand and Australia in 2013 before the 5-0 Ashes whitewash in Australia in the winter, and a first home series loss to Sri Lanka last month.
Warne described Cook's performance on the penultimate day of the second Test against Sri Lanka at Headingley as "the worst day of captaincy" he had seen at international level in almost 25 years.
|Alastair Cook as England Test captain|
|Matches: 23 Wins: 9 Defeats: 7 Draws: 7|
Vaughan, who captained England from 2003 to 2008, dismissed that criticism, but acknowledged Cook is under pressure.
He said: "If you're the England captain, you're a punchbag. You've got to take the punches for the team, and at the moment, he is taking a lot.
"I think the best thing that's happened is that he's had three or four days away. You can almost start afresh.
"I think the first innings of the series is massive. He will feel that is one where he has to score runs."
Vaughan says Cook has the "perfect opportunity" to find his form against an India seam attack he believes "looks average".
The former Yorkshire batsman added: "If you look at his most successful time as an England captain, it was in India.
"He averaged 80, scored three centuries and England won the series. He led from the front by scoring big, big runs. That is what he has to do again."
Vaughan later told the BBC: "He's got to go back to the old Alastair Cook - the old-fashioned way of playing, the old school, Geoffrey Boycott-style. Bat for time, bat as long as you can.
"Make the bowlers come to him. If he starts chasing it and pushing at the ball like he has done for a while now, Ishant Sharma and Varun Aaron could cause him a few problems."
Vaughan was also positive regarding England's own bowling attack and the general health of a team in transition.
"English cricket's fine," he said. "Yes, they need to find a spinner but with four good seam bowlers and a batting line-up that is probably going to be as deep as England have ever had, I see there is a lot of light at the end of the tunnel."
However, former England captain Kevin Pietersen predicted that Cook and his team would struggle against India, particularly following the loss of off-spinner Graeme Swann, who retired from international and first-class cricket in December.
Swann, who took four wickets against India in Chennai on his Test debut in 2008, became the sixth-highest wicket-taker for his country.
Writing in his column for the Daily Telegraph, Pietersen said it was the "genius" of Swann, rather than the tactics of the captain, that made the difference in previous Test series against India.
The Surrey batsman, who in February was informed he was no longer part of England's plans, wrote: "The Indians respect Swann hugely - they think he is a fantastic bowler and I know they are licking their lips about facing any other English spinner who bowls to them."