Steve Smith lays on the pitch after being hit in the head by a ball off the bowling of Jofra Archer (unseen) during play on the fourth day of the second Ashes Test match at Lord's Cricket on Saturday. AFP

Smith was in sight of his third successive Ashes hundred when, on 80, he turned his head away from an express delivery from England fast bowler Archer timed at 92.4 mph.

The star batsman fell face forward onto the ground and received several minutes of on-field treatment involving both Australian and England medical staffs amid concerns he could be concussed.

Smith lays on the pitch after being hit in the head by a ball off the bowling of Jofra Archer (unseen). AFP

He eventually got to his feet and then, after a prolonged conversation with Australia team doctor Richard Saw, decided to retire hurt with Smith receiving a generous round of applause as he walked back into the pavilion.

Australia were then 203-6, 55 runs behind England's first-innings 258.

World Cup-winner Archer, making his Test debut, had previously struck Smith on his unprotected forearm with another bouncer.

This innings saw Smith become the first batsman to post seven consecutive Ashes scores of fifty or more.

His half-century at Lord's followed the 30-year-old Smith's superb scores of 144 and 142 in Australia's 251-run win in the series opener at Edgbaston last week — his first Test since he completed a 12-month ban for his role in a ball-tampering scandal in South Africa that cost him the Australia captaincy.

Smith falls to the pitch after being hit in the head by a ball off the bowling of Jofra Archer (unseen). AFP 

Australia resumed on 80-4 after England's quicks had sparked a top-order collapse.

But star batsman Smith, fresh from twin hundreds in Australia's 251-run win in the series opener at Edgbaston last week — his first Test since completing a 12-month ban for his role in a ball-tampering scandal — was 13 not out.

Meanwhile Matthew Wade, who also made a hundred at Edgbaston, was unbeaten on nought as the skies started to cloud over, with the floodlights switched on.

Smith, who before play told former Australia captain turned Channel Nine broadcaster Mark Taylor he had slept in the bathtub at the ground while it rained on Friday, produced some trademark extravagant leaves.

But he also struck two stylish backfoot forcing shots for four off Stuart Broad.

Fast bowler Jofra Archer, fresh from taking his maiden Test wicket on Friday, came on in the fourth over.

Smith lays on the pitch after being hit in the head by a ball off the bowling of Jofra Archer (unseen). AFP

It was Broad, leading the attack in the absence of the injured James Anderson, who broke through when he had Wade edging low to Rory Burns at third slip for an innings of six that took 45 balls.

Paine was beaten on the outside edge by the express pace of Archer and some fine deliveries from Chris Woakes.

But when Woakes dropped short, Paine cut him for a well-struck four as the sun started to shine.

After 57 overs of right-arm pace bowling, England captain Joe Root finally brought on the recalled Jack Leach amid suggestions Smith could be vulnerable to left-arm spin.

Agence France-Presse