Reforms to simplify spelling have been a consistent source of argument since the beginning of the century.

The present proposals, which will give originally foreign words a more German nature, have been heavily fought over. Some say the changes are just cosmetic, while others say they go too far.

The changes will include foreign words being Germanised, turning Restaurant into Restorant, Crepe into Krepp, Ketchup into Ketschup and Spaghetti into Spagetti.

An agreement on the changes was due to be signed in Vienna on Monday by Germany, Austria and Switzerland. Schools in Thurigen and Bavaria were due to implement the changes at the start of term in August.

But the last-minute legal challenge could cost the publishing house Duden millions of marks in lost revenue and scupper what would have been a bestseller. They are to bring out the definitive new German dictionary, including the changes, in August. Every German-speaking home will need one.

Professor Rolf Groschner, 48, from the Friedrich Schiller University in Jena and his daughter, Alena, are furiously trying to block the reforms at the 11th hour in Germany’s constitutional court in Karlsruhe Professor Groschner claims the reforms would be an infringement of constitutional rights. He said language was “one of the conditions that allows the development of free speech”.

He referred to the handbook of public law by constitutional judge Paul Kirchhof, which stipulates that citizens have the right to protect themselves against controls and influence on language.

The issue is complicated further in that it is up to each of Germany’s 16 regions to start to adopt the changes.

In a bid to block Professor Groschner, the conference of regional education ministers, meeting last week in Bonn, said the implementation of the reforms could no longer be opposed because they adhered to the constitution.