Steve Smith has been praised for showing incredible bravery in his Ashes innings of 92 after copping a blow to the arm and sending a massive fright around Lord's when he was felled by a Jofra Archer bouncer.

Smith's battered and bruised performance at Lord's will live long in the memory after a spell that saw him advance to 70 before he copped a brutish delivery flush on his unguarded forearm.

He received immediate treatment from the Australian physio, and looked in some discomfort before getting his arm strapped with protective padding.

But the hostile spell was far from over as Archer, bowling at a searing pace, landed a bouncer beneath Smith's helmet grill on his neck, sending the batsman sprawling to the floor in frightening scenes.

Smith lay prone on the deck as medical staff from both teams hurried out to check on him.

After receiving treatment, Smith got back on his feet and left the field for assessment, receiving a round of applause from the majority of the Lord's spectators.

Smith returned to the field after retiring hurt for 40 minutes. ( AP: Alastair Grant )

"Steve was hit on the neck below the left ear," read a statement by Cricket Australia. "He was assessed lying on the pitch at the instructions of team doctor Richard Saw.

"Dr Saw made the precautionary decision to remove Steve from the field of play to have him further assessed under Cricket Australia's head impact protocol. Steve then passed his assessments and will now be monitored on an ongoing basis, as is routine."

After the dismissal of Peter Siddle 40 minutes later, Smith returned to resume his innings, again receiving applause as well as praise from broadcast commentators.

Smith passed a concussion test after leaving the field, but will be monitored in the following days. ( Reuters: Paul Childs )

However, a smattering of boos from within the crowd could still be heard on the broadcast.

Smith, David Warner and Cameron Bancroft have received boos throughout the tour of England summer from English crowds since their return to the international fold following their ban over the ball-tampering scandal.

Those boos did not sit well with some pundits, prompting a rebuke from a host of former cricketers.

Former England captain Michael Vaughan said Smith deserved "a standing ovation for that innings".

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Former Australia captain Mark Taylor, speaking on Channel Nine's broadcast, described the booing as "disappointing", but reserved praise for Archer's spell of bowling, saying it was one of the finest displays of pace you could see at Lord's.

"He doesn't try to hit anyone, he had a job to do and he did it very well," Taylor said.

Archer was bowling at speeds of up to 154 kph on day four.

Smith went on to make 92 before being trapped plumb LBW, looking out of sorts as he walked off the pitch.

Smith later went for scans on his arm and was cleared of a fracture.