Prasith suffers from a heart complaint and has had three bypass operations.

He was appointed to the Cabinet in November last year.

Surayud said the resignation had nothing to do with conflict in the executive and would have little effect on the administration.

Prasith's secretary Santi Sathippong explained the minister's heart problems and elevated blood pressure required rest. He recently underwent a medical in the United States, where he was advised to give up work.

Some have suggested Prasith resigned in anger at the refusal of the National Legislative Assembly to pass legislation he submitted and a lack of progress in other bills.

"Prasith has problems with his work, yes, but he is resigning because of his health," Santi said.

But Wallop Tangkhananurak, an Assembly member and associate of Prasith, doubted this.

"I was shocked he quit. He has a lot of patience and was trying his hardest," he said.

Wallop believed Prasith was depressed and frustrated at having to defend other ministers before the Assembly.

He said Prasith told him recently he was tired and worried and unsure of what to do.

Piyanart Srivalo

The Nation