A British judge on Friday sentenced a fraudster to three and a half years in jail after he tricked an elderly woman into paying him for a fake coronavirus vaccine jab.

Crown court judge Hannah Finch described 33-year-old David Chambers's actions as "shameful and despicable".

Chambers knocked on the victim's door in Surbiton, west London, on December 30, 2020, claiming to work for the state-run National Health Service (NHS), according to police.

The woman let him into her home, where he jabbed her in the arm with a "dart-like implement" before asking her to pay £140 ($194), which he said would be refunded by the NHS. 

He later returned and demanded £100 more.

Following a public appeal, the conman went on the run but was arrested in January.

The victim, who is now 93, said Chambers had worn fake NHS identification.

"I have never been subjected to such a deceitful and horrific crime," Kathleen Martin said in a statement released by police but expressed hope that others would not be deterred from getting vaccinated.

Maryam Arnott of the Crown Prosecution Service said after the ruling that Chambers had a "criminal history of exploiting elderly people" and took the opportunity to "cynically extract funds".

The pandemic has prompted a wave of scams exploiting public confusion over ever-changing UK government rules. 

ActionFraud, a national resource to report fraud and cybercrime, said in July it had received over 700 reports of people receiving scam emails and text messages purporting to be from the NHS and offering them vaccine passports.