Singapore's Prime Minister Lawrence Wong fired the leader of the opposition in parliament on Thursday, citing a criminal conviction and requesting the lawmaker's party to nominate a successor.

Wong's decision follows a parliament vote on Wednesday to declare Pritam Singh unfit for the role of opposition leader.

"Having considered the matter carefully, I have decided that Mr Singh's criminal convictions, taken together with Parliament's considered view of his unsuitability, make it no longer tenable for him to continue" as the leader of the opposition, Wong said in a statement.

There was no immediate comment from Singh.

Wong said he had sent a letter to Singh's party inviting them to nominate a replacement. The Workers' Party confirmed it had received the letter. (AFP)