Lord's went deathly silent and Steve Smith lay on the turf after copping a sickening blow to the neck on day four of the second Ashes Test.

Debutant Jofra Archer was in the middle of a frighteningly quick spell and had been testing the former Australian cricket captain with a barrage of seriously fast bouncers that had him hopping around the crease.

After already striking Smith with a blow some speculated may have broken his arm, the second session took a terrifying turn a short time later when Smith was unable to get out of the way of a searing Archer bumper.

On 80, Australia's best batsman tried to get out of the way of the ball but it was simply too quick and the six-stitcher crashed into his neck.

Batting at the Nursery End, Smith fell to the ground immediately just to the right of where he was taking guard on the edge of the pitch. Everyone knew it was serious and while the crowd was getting rowdy in the middle of Archer's incredibly hostile spell, spectators at the ground went quiet when Smith hit the deck.

England's players rushed to his side and umpires immediately called for assistance as Australia's medical staff ran out onto the ground.

Smith managed to get his helmet off straight away and then rested his head on his forearm. He was able to roll onto his back and tried to take his gloves off but couldn't, then rolled onto his side as Australian team doctor Richard Saw arrived on the scene.

On Twitter, former Australian women's captain Lisa Sthalekar said "I feel sick" having seen the scary incident and Channel 9's Ashes anchor Todd Woodbridge called it "gut-wrenching viewing".

Play came to a halt for a lengthy amount of time as the medicos showed great care for Smith. He eventually got back his feet and continued to be assessed by the doctor.

After the lengthy break Mr Saw made the precautionary decision for Smith to go off and he was escorted from the middle to undergo testing as everyone at Lord's gave him a standing ovation.

Smith was evaluated for symptoms of concussion and assessed under Cricket Australia's head impact protocol. There was talk among commentators Australia may need to use a concussion substitute for the first time in Test cricket history.

Teams have the option of replacing a player who suffers a head or neck injury during the match and is diagnosed with concussion or suspected of being concussed.

But Smith passed all the necessary tests and was cleared to keep playing and it wasn't long before he was back in the middle when Peter Siddle was caught behind.

His comeback was incredible. Smith slammed Chris Woakes over the leg side for four then punched him off the back foot to the cover boundary very next ball.

Smith was on the verge of scoring his third century of the series, making his way to 92 before a somewhat comical misjudgment ended his stay at the crease. He shouldered arms to a Woakes delivery but he didn't realise it was an inswinger that darted back into him.

The ball struck him on the pads right in front of the stumps and there was no saving the 30-year-old as the umpire raised his finger, ending a sensational innings.

Smith will be monitored on an ongoing basis.

He wasn't wearing the neck guards on his helmet that were introduced after Phil Hughes' death and doesn't wear them because he finds them uncomfortable.