Andre Lundt spokesman for the Berlin Federal Building Corporation: "We've had a lot of negotiations with Mr Foster"

The multi-million dollar renovation by the leading British architect Lord Foster, which won awards at the time of opening, has come in for serious criticism in recent days.

The German authorities are threatening to withhold the final instalment of Lord Foster's fee for the job until a string of complaints have been addressed.

They say they will not pay up until they are satisfied.

Sir Norman was awarded with a top honour

Members of the Berlin Federal Building Authority and architects representing Lord Foster are addressing more than 40 complaints.

An authority spokesman said none of the five major faults meant the structure was dangerous.

But Andre Lundt added: "We are talking about services that were contractually agreed upon, not aesthetic consideration."

The German authorities say there is water penetration due to uneven stone near the base of the spectacular glass cupola and in one area, the wrong concrete has been used.

Under German rules, the federal authorities are required to hold back up to 5% of the contract's value until they are absolutely satisfied, and unless the two sides can agree, Lord Foster could stand to lose just under $2m.

On a brighter note, most of the minor complaints have been rectified.

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The wobbly press gallery, which had irritated photographers, has been secured, and many of the acoustic problems bothering MPs are to be tackled, according to a spokeswoman for the Bundestag.

But there was also a whiff of farce: committee members from the Green Party and the liberal FDP had complained that tiny gaps in the internal wall cladding led to a glimmering effect which gave politicians headaches in meetings.

Lord Foster's company has declined to comment on the controversy.

He was awarded the Pritzer Architecture Prize, a top honour, for his work on the building, which features a huge glass dome with walkways.

The architect is famous for multi-million pound projects all over the world, from Germany to China, including airport terminals, skyscrapers, museums and public buildings.

Major tourist attraction

The Reichstag, former home of the German parliament, reopened in April in the centre of unified Berlin.

The government's move back to the Reichstag was part of the gradual shift of the German capital from Bonn to Berlin, a move envisaged since the reunification of the former West Germany and East Germany in 1990.

Since it reopened, the building has become one of Berlin's leading tourist attractions.