The inventor of the Hot Spot thermal imaging replay system has apologised to England for the manner in which Jonathan Trott was given out by the third umpire on the second day of the first Ashes Test at Trent Bridge.

England collapsed to 11 for two in their second innings against Australia on Thursday after Joe Root and Trott, out for a duck, fell to successive Mitchell Starc deliveries.

Root chose not to challenge his decision, even though replays suggested he would have been reprieved, as technology indicated he had made no contact with the ball before being caught down the leg side by wicketkeeper Brad Haddin.

Next ball, Pakistani on-field umpire Aleem Dar initially gave Trott not out in reply to Australia's lbw appeal.

Australia challenged his decision and South African third umpire Marais Erasmus overturned Dar's original verdict despite the fact the side on Hot Spot image of the dismissal, which might have confirmed whether Trott had in fact edged the ball first, was not available.

Angry England coach Andy Flower demanded an explanation from match referee Ranjan Madugalle, the International Cricket Council's most senior official, over the lbw dismissal while bowler Jimmy Anderson called it "very frustrating".

Host broadcaster Sky explained the Hot Spot camera missed the Trott wicket as it was replaying Root's exit a ball earlier. Images cannot be replayed and played at the same time.

Meanwhile the inventor of the Hot Spot system apologised for an "operator error".

"Here is the absolute truth from our perspective in regard to the Trott incident," Warren Brennan told the cricinfo website.

"It was operator error. My operator did not trigger the system in order to cater for the Trott delivery.

"Instead the operator sat on the Root delivery in order to offer a replay from the previous ball and did not realise until it was too late that he should have triggered the system for the Trott delivery as the priority.

"Simple mistake, something that anyone could have made but my Hot Spot operator has worked on the system since 2007 and to my knowledge this is the first serious mistake he has made."

Hot Spot is provided by an independent company for both Sky and the ICC.

Dar seemed bewildered when told he had to give Trott out and the batsman himself was aghast.

Later the "Snickometer", which is not part of the official review system, revealed a thin edge.

But neither this nor the square leg Hot Spot angle were available to Erasmus who was able to view only shots picked up from behind the bowler's arm, which showed no mark on the bat.

Normally the third umpire overturns a decision only if there is conclusive evidence of an error.

England eventually reached 80 for two, a lead of 15 runs, at stumps on the second day.

But they left Trent Bridge convinced they should have been better placed after another contentious third umpire decision went against them.

Australia last man Ashton Agar was on six, with the tourists 131 for nine -- 84 runs behind England's first innings 215 -- when he survived a desperately close stumping appeal from home wicketkeeper Matt Prior off the bowling of off-spinner Graeme Swann.

The 19-year-old Agar, on his Test debut, went on to make 98 -- the highest score by a Test match No 11 - in a record last-wicket stand of 163 that took his side to 280 and a first innings lead of 65.

England and many other observers were convinced Agar was out.

"It was a shocking decision," said former England captain Bob Willis, now a Sky Sports pundit who last month caused a furore by claiming England had tampered with the ball during the Champions Trophy to gain reverse swing.

"It cost England dearly. That was a terrible decision and it may lose the game for England."

Willis, one of only four England bowlers to have taken 300 Test wickets, was also scathing about Trott's exit.

"It was staggeringly bad -- unbelievable," he added.