A Banksy collector and expert believes a mural that appeared at Extinction Rebellion’s Marble Arch base overnight is an authentic piece by the Bristolian street artist.

John Brandler, who owns a dozen pieces by Banksy is convinced the artwork – which features the slogan “From this moment despair ends and tactics begin” next to a young girl sitting on the ground holding an Extinction Rebellion logo – is an original because of its execution and theme.

The art dealer and gallerist said: “I’m convinced about the one in London for two reasons: it’s a topic that he would support, and it’s a continuation of the Port Talbot piece that appeared in December 2018.

“The name in the corner is not important, the signature is the work. And this is a Banksy. It’s a wonderful statement and a beautiful piece.”

The work appeared at the site which had been occupied by climate activists since protests began in the capital almost two weeks ago.

A spokesperson for Westminster council confirmed the work was being investigated but had not been authenticated yet. “We’re aware of the possible Banksy which appeared in Marble Arch overnight. Our officers are looking into this,” he said.

Banksy has not confirmed whether the painting is legitimate, and his press team did not respond to a request for comment.

Brandler said the Marble Arch piece was much more convincing than another possible Banksy that appeared on a primary school wall in Maidstone over the Easter holidays. “There was a suspected Banksy on a school in Kent, but it was obviously not one,” he said.

“‘If we don’t act now, what happens?’ That’s what this little girl is saying. Now is the disaster; now is the time to act. [The Marble Arch piece] is similar to the boy with the flag during the 2012 Olympics.

“The only thing I don’t like is that the top of the girl’s head is the same height as the concrete block. Visually, it jars.”