The Constitutional Council announced the decision to deprive Mr Lang, 54, of his seat on the basis of a complaint by his conservative opponent. It banned him from standing in elections for 12 months from 28 March, the date of his election. Since a by-election in his Blois constituency in the Loire Valley has to be held within three months, he cannot regain the seat in this legislature.
The move hit one of President Francois Mitterrand's inner circle. Opinion polls regularly find Mr Lang to be one of France's most popular politicians, especially among the young.
The Constitutional Council, headed by Robert Badinter, a former Justice Minister and an intimate of the President, found that Mr Lang's campaign had cost 589,816.65 francs ( pounds 67,560), 13 per cent over the legal 500,000-franc maximum. The most original campaign expense was the organisation of three excursions to Paris for Blois pensioners. After visiting the Louvre, the old people were personally entertained by Mr Lang at his ministry in the Palais Royal.
Mr Lang is a man who leaves few people indifferent. He was Culture Minister in all the Socialist governments from Mr Mitterrand's first election to the presidency in 1981 until the Socialists' defeat in March. He added Education to his portfolios and was the highest-ranking minister in the last government.
His fans consider his youthful allure - with striped Thierry Mugler shirts and stylish suits - a refreshing change from the staid establishment look. His detractors regard him as superficial and partisan, someone who used his position to promote cronies.
Mr Lang, formerly a university law lecturer, was 'parachuted' into Blois - where he has since been elected mayor - in 1986, when a safe constituency was sought for him. In regional elections last year, when the Socialists were all but routed, he gained the highest score of any of the party's candidates.
Mr Lang is the third deputy to be stripped of his seat by the Constitutional Council. Last month, two others, one Gaullist and one member of the centre-right Union for French Democracy (UDF), were also disqualified.
Bernard Tapie, the former Minister for Towns, successfully contested a similar complaint and the Constitutional Council ruled in his favour last week. But Mr Tapie's parliamentary immunity was lifted by his fellow deputies on Tuesday to allow investigations into his business affairs.
With typical verve, Mr Lang said the finding against him would be 'a springboard for a new battle'. Michel Rocard, the Socialist Party First Secretary, regretted the decision, but said: 'Jack Lang should have been more careful.'