Glenn McGrath blasted a controversial decision to disallow a crucial 'catch' by Australia's Mitchell Starc in the second Test at Lord's as a "disgrace".

It appeared England opener Ben Duckett had fallen for 50 when he miscued Cameron Green to fine leg, where Starc held a low catch only to ground the ball while still sliding on the turf.

Under cricket's laws, a catch is only completed when the fielder has "complete control over the ball and his/her own movement" and cannot touch the ground before then if a dismissal is to be completed.

The on-field umpires referred the decision to TV official Marais Erasmus, with the experienced South African ruling in Duckett's favour, much to the disgust of retired Australia fast-bowling great McGrath.

"I'm sorry, that is the biggest load of rubbish I have ever seen," McGrath said while commentating for BBC Radio's Test Match Special on Saturday.

"He (Starc) has got that ball under control. That ball is under control.

"I've seen everything this game has to offer. If that is not out, then every other catch that's ever been taken should not be out. That is a disgrace."

But Marylebone Cricket Club, the owners of Lord's and cricket's law-makers, insisted Erasmus had made the correct decision.

"Law 33.3 clearly states that a catch is only completed when the fielder has 'complete control over the ball and his/her own movement'," the MCC said on their Twitter account.

"The ball cannot touch the ground before then. In this particular incident, Mitchell Starc was still sliding as the ball rubbed the ground, therefore he was not in control of his movement."

England ended the fourth day on 114-4, with Duckett 50 not out, and needing a further 257 runs to reach a victory target of 371 in their bid to level the five-match series at 1-1.