Sri Lanka, Jan. 20 -- Dutch lawmakers will vote on Tuesday on a first step towards requiring King Willem-Alexander and his family to pay income tax, a plan that would require an amendment to the constitution.

"A blue envelope for blue blood," Sandra Beckerman, a lawmaker from the left-wing Socialistische Partij, said during a debate on the issue on Thursday evening, referring to the blue envelope in which tax letters arrive in Dutch mailboxes.

The initial motion, expected to be put to the vote on Tuesday, requires a simple majority to pass. It asks the government to propose a constitutional change, which would then ultimately need backing from two-thirds of both houses of parliament.

Such a proposal appeared to have sufficient support ...