Roland and Jayne Pearson were granted permission by Lancaster City Council planners to erect the conservatory at the back of their home in Grosvenor Court, Carnforth in July 2005.

But they failed to heed a condition requiring them to install obscure glazed non-opening side windows, which was imposed to protect the privacy of their neighbours.

The council began enforcement action in September 2005 and the couple were earlier this month finally prosecuted at Lancaster Magistrates Court for failing to comply with a breach of condition notice after several reminders and warnings.

They pleaded guilty and were each fined £100 and ordered to pay £100 towards the council's costs.

If they do not comply with the condition they could be prosecuted again and as a last resort the council even has the power to carry out the work and send them the bill.

After the court hearing, Mr Pearson, 48, said the council's stance was "unfair".

He said that in January this year he had attached obscure glazed plastic sheeting to the window and bolted it down so it could not be opened.

But the council is insisting he must replace the window. Mr Pearson says that would involve replacing the entire 6ft x 3ft panel at a cost of almost £3,000.

The father-of-two said: "I'm an unemployed brickie and my priority is to get food on the table for my family, but it will cost me a fortune to do what they are asking.

"Nobody can see in through the window now and it cannot be opened. Originally I left it with the manufacturer and that's how it came, but the neighbours have not complained and one even said I could put the Brighton Pavilion up and he would not be bothered."

Coun Eileen Blamire, the council's cabinet member for planning enforcement, said: "Conditions are imposed on planning permissions for important reasons such as, in this instance, to protect the privacy of neighbours. Compliance with planning conditions is not an 'optional extra' for developers and we will rigorously pursue those who fail to do so."

A council spokesman said the authority had received a complaint about the conservatory.

He added that the actions taken by the couple in January were "temporary measures which could very easily be removed."