Sunderland City Council took fruit-and-veg trader Steven Thoburn to court for refusing to convert to European-approved metric measures and selling his goods in kilos and grammes.
Mr Thoburn, 36, who appeared before Sunderland magistrates, now faces a maximum fine of £1,000 on each of two offences and court costs estimated to run up to £60,000.
He was convicted of breaching the Weights and Measures Act 1985 in a hearing which district judge Bruce Morgan said centred around the "most famous bunch of bananas in legal history".
In his ruling, the judge said he was aware that regardless of the verdict the case would be going to appeal.
"It has been made clear to me that despite the decision of this court the matter will be taken elsewhere," he said.
Mr Thoburn, who denied the charges, attracted huge support for his fight against the weights and measures legislation.
His case was taken up by the UK Independence Party (UKIP) which believes the outcome could decide the future of the nation's weights and measures system.
The legal battle started last summer when local authority trading standards officers seized Mr Thoburn's scales.
Metric campaigners
Mr Thoburn and his fellow trader Neil Herron, a fishmonger who has also been ordered to change his scales, were described by some as "metric martyrs" and a legal expenses fund was set up.
UKIP solicitor Tony Bennett said at the time: "This is not about one trader taking on his local council, the implications are huge.
"The eyes of Europe will be on Sunderland while this case is in progress because it will decide the future of our traditional weights and measures."
Mr Thoburn, who is now using dual measure scales, gathered support from shoppers to hand over a 5,000-signature petition to Downing Street.
His campaign was also backed by a number of celebrities from the entertainment world.
European regulations
Europe's 1994 Units of Measurements Regulations came into effect on 1 January last year.
Trading standards officers seized Mr Thoburn's scales from his market stall in Southwick, Sunderland, on 4 July 2000, following the introduction of the directive.
Sunderland City Council said the father-of-two had failed to comply with a notice issued by its officers to convert to metric measures at his stall.
But Mr Thoburn's legal team believes British traders have a legal right to use imperial measures under the provisions of the 1985 Weights and Measures Act.
They have argued that the government gave assurances that stallholders could continue to use imperial measures.