Pakistan captain Salman Butt defended his side doing without the controversial Decision Review System (DRS), during a batting collapse in the first Test against England, by saying the technology on offer was "not a hundred percent".

Pakistan's Butt voices DRS doubts

The tourists were in the midst of a slump that saw them decline to 47 for six on the second day at Trent Bridge here on Friday when Azhar Ali was given out caught behind for 14 in one of five wickets for James Anderson.

However, some replays indicated he had failed to make contact.

Under DRS, both sides are allowed two unsuccessful challenges before they are barred from requesting a referral for the remainder of an innings.

But, in order not to delay the game, they must make it clear within a matter of seconds of the umpire's original verdict whether they wish to call on DRS.

It appeared from a distance the inexperienced Ali, in only his third Test, decided against a referral that might have gone in his favour after consulting non-striker and fellow novice batsman Umar Akmal.

Butt, speaking after stumps, said that wasn't the case and questioned the accuracy of both the Hawkeye ball-tracking system and the Hotspot device, akin to a thermal imaging camera, which is used to detect thin edges.

"This technology, Hawkeye is not 100 percent and neither is Hotspot. When the ball hits the sticker on the side (edge of the bat) it doesn't leave a mark," Butt told reporters.

A further complicating factor in this match is that spectators in the ground are seeing a replay on a giant screen at the same time as the third umpire is coming to his decision on the basis of looking at the same image.

In previous series, fans at the game have often been kept in the dark.

"If the batsman knows he has hit the ball there's no point taking a chance because if it's up there on the big screen it's pretty embarrassing," opening batsman Butt added.

"I think it was very honest of Azhar Ali, good sportsmanship. He edged it and he walked straight away."

Butt said he'd been alerted to what he believed to be a deficiency in Hotspot during Pakistan's 3-0 series loss in Australia earlier this year.

"In January we played Australia in Hobart and it twice happened to Michael Clarke. We heard a big noise and the Snicko showed there was noise as well when the ball crossed the bat but there was nothing on the Hotspot.

"Every bat brand has a side-sticker (on the edge). It shows some time and it doesn't show. That means it's something to do with the stickers, maybe."

The Snicko device, which tries to detect whether a batsman has edged the ball on the basis of sound waves, is not being used as a decision-making tool in this four-Test series because officials say it takes too long to provide a definitive conclusion.

But it was Hawkeye that was in Butt's sights. "Hawkeye is not a hundred percent trustworthy," the 25-year-old insisted. "Every meeting we have with the officials, they say it's not a hundred percent."

Anderson, who exploited the swing friendly conditions to finish with five wickets for 49 runs from 20 overs on Friday, said of DRS: "It's trickier than it seems, and something we're having to get used to."

Pakistan will resume on 147 for nine in reply to England's first innings 354, a deficit of 207 runs, on Saturday.

But Umar Gul's 30 not out took them to within just eight runs of avoiding the follow-on, albeit with only the one wicket standing, when bad light forced an early close on Friday.

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