Steve Thoburn, the market trader prosecuted for selling bananas by the pound, was yesterday found guilty by a court in Sunderland of breaching the Weights and Measures Act 1985.

Handing down a six-month conditional discharge, the judge said that it was obvious that "Parliament has surrendered its sovereignty to the primacy of European law" and when the then prime minister, Sir Edward Heath, had signed the Treaty of Rome, he had effectively "signed away the imperial system".

Alan Duncan, the shadow trade and industry minister, said that the council had "persecuted a decent bloke for no good reason", and urged the council not to pursue its estimated £75,000 legal costs.

A spokesman for the Save the Scales campaign said that the ruling was proof that the Queen no longer has sovereignty and that Great Britain no longer exists as a legal entity".

The legislation under which Thoburn was prosecuted, the Units of Measurement Regulations 1994, were unanimously agreed in just 21 minutes by a committee of 14 MPs. It was a Tory minister, Ian Taylor, who took the regulations through the committee. The case has been cited as an example of the lack of parliamentary scrutiny of so-called delegated legislation.