Woolwich Crown Court is a main centre for high-security cases.
Judges have voiced concerns about plans to build prefabricated courtrooms next to a court that hears high profile terrorism and organised crime trials.
BBC News has learned that the portable units will be assembled in a car park at London's Woolwich Crown Court.
More space is needed because caseloads have risen in England and Wales.
The Council of Circuit Judges said it was concerned the units might not provide the best facilities for vulnerable witnesses and jurors.
Woolwich Crown Court is currently one of the main centres for hearing high-security cases that require armed protection.
BBC home affairs correspondent, Danny Shaw said the Court Service was spending £6.8m on five modular courtrooms.
In the past five years, the caseload in Crown Courts across England and Wales has risen by more than 11%, and by 23% in London.
The new prefabricated courtrooms will be built in a factory before being assembled in the grounds of the court.
They are due to be ready by next September and are designed to last for between five and 10 years.
Recently Woolwich Crown Court has dealt with cases such as a £53m raid on a Securitas depot in Kent, the plot to kill thousands of people by blowing up planes with liquid bombs, and a plot to set off car bombs in London and Glasgow Airport.
Last week the UK Supreme Court opened at Middlesex Guildhall, which was refurbished and refitted for nearly £59m. The President of the Court, Lord Phillips, said he hoped it would last for at least 100 years.