Vandals wrote the graffiti along with slogans opposing plans to build eight wind turbines overlooking the site of one of the most important battles in British history.

Power company Eon is working with the trustees of Kelmarsh Hall estate on plans to erect the 120-metre high wind turbines.

It yesterday criticised vandals who defaced a visitor information board at Naseby's battlefield which is about two kilometres from the proposed wind farm site.

The information board at Prince Rupert's viewpoint was defaced with graffiti sometime last week and has since been cleaned.

Project developer Daniel Badcock said: "The Naseby Battlefield Project are the victims here as they spend a lot of time and money making this important battlefield accessible to the public.

"This will be a setback for them. It is hugely disappointing this happened while most people were enjoying the Kelmarsh Family fun day."

The information boards were put up by the Naseby Battlefield Trust, a charity devoted to maintaining the historic site of the battle which brought in the advent of parliamentary democracy.

The English Civil War battle brought an end to the Royalist's hopes after their defeat by the Parliamentary troops led by Sir Thomas Fairfax and Oliver Cromwell.

Historian Martin Marix Evans, who is deputy chairman of the Naseby Battlefield Trust, said the graffiti was insulting, adding that the vandals had displayed "total ignorance of history".

He said: "It's a really revolting thing to do and there's no place for it. What it was meant to represent, I'm not too sure. It just shows total ignorance."

Wendy Westall, a member of the Stop Kelmarsh Wind Farm Committee, said: "We are openly against the wind farm but would certainly not condone anyone displaying this sort of behaviour. I find it absolutely unbelievable. Nothing excuses this sort of hooliganism."