Official papers made public yesterday show civil servants initially reacted with amusement when in 1963 Victor Depaepe tried to revive a royal charter granted in 1666 by Charles II, allowing 50 fishermen from Bruges to fish in English waters.
But, according to the files released to the Public Record Office, their merriment ended when it emerged that his claim could be legal. Ministers were warned that they might even have to change the law after his case was taken up by the leading QC, Sir Frank Soskice.
When Mr Depaepe, a Bruges city councillor, set out to "invade", he made no secret of his intentions, sending telegrams to both the Queen and the then prime minister, Harold Macmillan, informing them of what he was doing. He declared that he was "no pirate" and made it clear that he hoped to be arrested for illegal fishing so that his claim could be tested in the courts.
Despite being informed of both when and where he was arriving - off Seaford, East Sussex, on 10 July 1963 - the Royal Navy sent a gunboat andminesweepers to apprehend him in a blaze of publicity.
Afterwards, Mr Depaepe wrote again to the Queen in his broken English expressing gratitude for the way he had been treated, without even his nets or catch being seized. "Heart-whole thanks for Her Majesty's attitude that means for me a royal present, symbolising the British fair play," he declared.
But even then, behind the scenes there was concern, with civil servants at the Ministry of Agriculture pressing the BBC not to treat his claims seriously. And as the saga dragged on the concern grew.
Meanwhile, Sir Frank advised that the case should be heard in the High Court rather than before Lewes magistrates in Sussex. That set alarms bells ringing, and the Minister of Agriculture's lawyers warned that the chances of successfully arguing that the charter had been overridden by later legislation were "good but not overwhelming".
They advised that if the case went to court the only way out might be new legislation specifically repealing the charter - which would be tantamount to admitting Mr Depaepe had been right all along.