Bristol may have gained 100 extra Banksys for three months through the summer, but the sad fact is that as a result of the show we have ended up with fewer permanent intact Banksys than we started with back in June.

Anybody could have predicted that some elements in the reactionary street art community, from which Banksy hails, would react badly to the idea of the covert artist finally making it to the relative conformity of an art gallery-based retrospective exhibition.

Within days of the show's opening the first incidents of defacement for some of Bristol's most iconic pieces of Banksy began.

The most notable incident saw vandals throw blue paint over the famous Banksy at the bottom of Park Street – which depicts an adulterer hanging from a window.

The damage appeared to have been carried out using a paint gun – of the kind used by paint-balling attractions.

At first the city council tried to clean it up, but it quickly became clear that removing the blue paint would destroy the Banksy.

When the blue paint vandals struck again the decision was made to abandon any attempt to remove the unwelcome addition to the piece.

But this isn't the only Banksy street art to have been damaged during the course of the summer.

The equally famous Mild, Mild West piece in Stokes Croft – which features a teddy bear throwing a petrol bomb – was also splattered with blue paint.

The figure of a police marksman with a child, on Park Row since 2007, has also been defaced with a protest flag inexpertly added to the child's hand.

The figure of actor Michael Douglas in Falling Down

at Junction 2 of the M32 has had the head scrawled away almost entirely.

Even an unconfirmed Banksy stencil of a young Michael Jackson, which appeared in Easton shortly after the singer's death this summer, has now been daubed with the words "Banksy: Property of the State".

"It's very sad, to see that people feel the need to deface these works of genius," says Councillor Simon Cook, deputy leader of the city council and cabinet member for culture, sport and capital projects.

"We are now looking into ways to preserve the city's existing Banksys more effectively – which may eventually mean having clear plastic screens placed over them."

But Paul Barnett, acting head of Bristol's museum service, is less downbeat about it.

"I think it's fascinating in a way," he says. "I think it's saying something very interesting about the nature of street art. It's not permanent – it never can be. But the whole notion of graffiti-ing graffiti is intriguing.

"I'm sure that Banksy would just have a bit of a laugh about it.

"The interesting thing is that people have been able to differentiate between the street art and the work that Banksy has exhibited in the museum. The work in here has been treated with much more reverence – thankfully nobody has tried to add their own artistic contribution."