A pedantic vandal who daubed graffiti on a Hampstead Heath sign to correct its wording has been rounded on by the City of London.

The mystery Heath visitor took issue with a notice informing the public a patch of grass was “being sown with wild flowers” and made an addition in marker pen saying: “If they are sown they are not wild”.

After a photo of the altered sign circulated widely on Twitter this week – with users of the social networking website branding it an act of “middle class graffiti” – the City of London Corporation said it was a clear breach of Heath byelaws.

A spokesman said the sign must now be replaced – costing resources “which could have been spent on improving another area of the Heath”.

He added: “We are happy to answer any questions the public may have on any issues – but vandalising the signs is not the way to go about it.”

With so many spreading the snap around Twitter, Heathman was unable to trace the original photographer, but Heath bosses said the notice was located near the cafe in Parliament Hill and would cost about £100 to repair.

Meanwhile, horticultural experts have claimed there was nothing wrong with the phrasing in the first place.

Guy Barter, chief horticultural advisor of the Royal Horticultural Society, said: “The term wild flower is used rather loosely, but it is possible to sow seed of British wild flowers as these are widely offered.

“The wording would be acceptable if British wild flower seed, that had not been too long in cultivation, was sown.”

Meg Game, an ecologist at Hampstead Heath, said: “We source the seeds from a firm which harvests them from the wild in Britain before duplicating them in a nursery – so they are designated ‘wild flowers’.

“Many of the wild flowers in the seed mix would have grown naturally on the Heath more than a hundred years ago.”