Kevin Pietersen believes there is an "underlying current of unhappiness" within the England team following their first Test series defeat to Sri Lanka on home soil.

England suffered a 100-run defeat to Angelo Mathews' side at Headingley on Tuesday to lose the two-Test Investec series after the tourists escaped with a draw in the opening match at Lord's.

This latest setback prompted Pietersen, whose central contract was terminated by the England and Wales Cricket Board after the Ashes whitewash in Australia last winter, to suggest in his Daily Telegraph column that all was not well within the England camp.

He wrote: "E ngland were in winning positions in both matches but blew it, and I believe that is a symptom of the senior players being very unsettled. They are not turning up for the captain or coach.

"(Stuart) Broad and (James) Anderson looked jaded at Headingley. Why? Why was Jimmy so emotional? We have lost a lot of matches in the past but he has never shown such emotion. He has been through tougher times and suffered lower moments but never been in tears before.

"It says to me there is an underlying current of unhappiness. The Australia tour was hard. We were beaten up due to a lot of reasons I cannot go into right now, and some of the senior players are still suffering, with the result that they are struggling to offer leadership in the dressing room."

Pietersen, who made an unbeaten 24 in Surrey's eight-wicket NatWest T20 Blast victory over Hampshire on Friday, went on to praise the manner in which England's younger players have acquitted themselves over the past fortnight or so.

He added: "It was absolutely fantastic to see Gary Ballance, Sam Robson, Joe Root and Moeen Ali scoring hundreds.

"Add Ben Stokes and (Jos) Buttler to the mix and suddenly England will have a team mostly made up of young cricketers without any baggage, taking to Test cricket quickly and being successful."

Pietersen added Alastair Cook's form with the bat and his captaincy of the England team is a worry, with the left-handed opener failing to score a century in his last 24 Test innings.

"Cook's form will be affecting him," Pietersen said. "I have played with Cooky long enough to know that when that happens he becomes very quiet and introverted. He struggles to handle it. We have all been through it and it does play on your mind.

"Captaining the side without any runs plays on your mind too. I saw in Cook at Headingley the same look Andrew Strauss had when he played his last Test at Lord's. He was a rabbit in the headlights. It was a shame to see Cooky looking that way."

Glenn McGrath thinks Cook should remain as skipper, if only for the reason that there is no other obvious candidate to replace him.

McGrath's former Australia team-mate Shane Warne believes the left-hander needs to relinquish the captaincy or take a break from the game in the wake of England's sixth defeat in seven Tests.

But the former pace bowler has a different opinion, however, as he said: "I've always said, you replace a captain if he steps down or if there's someone better to replace him with.

"If there's no one that's going to do a better job, and he's happy to do it, then the captaincy should stay with him."

However, McGrath also thinks that Cook needs to freshen his approach as skipper.

"I like Cooky, and I have a lot of respect for him," McGrath, who is Australia's second highest wicket-taker of all-time with 563 Test scalps, added on

"But England are fairly set in their ways.

"They seem to have plan A and then plan B is nearly non-existent - it's stand back and wait for the opposition to make a mistake.

"I think in this day and age you have to be a little bit more innovative and think on your feet a little bit more."