A Belgian appeals court has ruled that former King Albert II is to be fined €5,000 (£4,370) a day if he refuses to undergo a DNA test.
In February, he refused to undergo such a test in a case aimed at proving he fathered a love child in the 1960s.
A court in Brussels ordered the 84-year-old ex-monarch to provide a saliva sample in three months or risk being presumed to be the father of Delphine Boël, 50.
The ex-king denies the paternity claim.
Rumours the king had an illegitimate child first emerged in 1999 in an unauthorised biography about his wife. The allegation prompted a royal scandal and enduring media gossip in Belgium.
Ms Boël first alleged on the record that King Albert was her biological father during a 2005 interview.
Her mother, Baroness Sybille de Selys Longchamps, has claimed the two had an affair between 1966 and 1984 when he was still Prince Albert of Liège.
However, following his older brother’s death in 1993 at 62, King Albert unexpectedly came to the throne.