"Doctor Mwanawasa had been making steady progress until last night when his condition suddenly changed and required intervention," said Vice President Rupiah Banda. The president suffered a second stroke in June.

Mwanawasa is currently in a hospital in France.

Speculation that Mwanawasa, 59, may be unable to return to work has prompted demands that doctors examine him and decide whether he is fit to remain president.

If parliament decides he is incapacitated Mwanawasa, could be forced to step down. Banda would take over as acting president and elections would be called.

"The team of doctors have reported that the intervention was successful but decribed the condtion of his excellency the president as serious," said Banda in the statement.

"The nation will be informed in due course on the future developments on the condition of his excellancy the president."