After travelling to Lille along the 140-mile TGV Nord Europe route, the French President congratulated the railway, SNCF.
The line will be extended to the tunnel mouth near Calais in September.
However, he added that while it was wonderful the trains would be able to run so fast - their top speed is 300kph (186mph) - on the French side, travellers to England were in for a shock in 1994.
'They will race at great pace across the plains of northern France, hurtle through the tunnel on a fast track and then be able to daydream at very low speed, admiring the (English) landscape and the countryside . . . until the day when someone over there in London decides to harmonise the way things are done on the continent and on the island,' he said.
It was an aside in a speech in which Mr Mitterrand showed genuine pride in the achievements of the French.
He was quick to emphasise the enormous benefits to the region of the line from Paris to Lille, which will eventually cut the journey between the two from more than two hours to
London could be reached within two hours and Frankfurt in three, attracting industry to the area.
An SNCF official on the inaugural train asked journalists about the UK rail link's progress. He said: 'I understand you still have a caravan going round Kent villages asking people what they think of the line.
'I hear that the caravan goes quite fast.'
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