The picture, which appeared on a fire exit from the Globe Hotel’s staff car park in the early hours of Monday, resembles the un-named artist’s trademark style.

But in a brief e-mail exchange Pest Control – which is his only point of contact – said: “This is not by the artist Banksy. Thanks for checking.”

The portrait of Shakespeare which has appeared in a doorway in King’s Lynn (Image: BCKLWN)

It came after one expert also cast doubts over the provenance of the piece, saying had flaws which would not be seen in a genuine Banksy.

Norwich-based urban art dealer Lee Webster said: “I’ve followed his work, I know his people. It’s a good stencil but it’s got a flaw.

“If you look at the arm there’s a gap. Banksy wouldn’t do that, if he did something he’d make sure that didn’t happen.

The gap in the arm, which one expert said was a tell-tale sign the picture was not a genuine Banksy (Image: Chris Bishop)

Lee Webster with some of the Banksys on show in Norwich last summer (Image: Denise Bradley)

“Another giveaway is if you look at the hand, if you look at the fingers they’re really good, if you look at a Banksy they’re like sausages, they’re not his strong point.”

Mr Webster, who curated an exhibition of Banksy’s work in Norwich last August, added: “It’s always interesting when something like this crops up, people always think it’s a Banksy but he’s not the only stencil artist out there.”

As sightseers flocked to see the piece, the true identity of the artist remained a mystery.

But the doorway in which it has appeared is around the corner from the medieval Guildhall of St George.

Council workers cutting sheets of plastic to protect the artwork (Image: Chris Bishop)

The stencilled portrait of the Bard has “Shakespeare woz ere” sprayed next to it.

Supporters of the guildhall revamp claim it is one of few which survive which can claim that the nation’s greatest playwright took to their stage.

But only scant circumstantial evidence exists, as West Norfolk Council looks set to borrow £16m towards the cost of a £30m revamp.

The car park, within a derelict building, is owned by hospitality chain JD Wetherspoon, which said it would wait to see whether the work was a genuine Banksy before deciding what to do with it.

West Norfolk Council has covered the piece with clear plastic, while a security guard is also watching over it.