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.
His lawyer, Guy Hiernaux, told reporters that he believed the former monarch would "submit to this test, since he has the guarantee that it will remain confidential".
"It depends on him," said another lawyer, Alain Berenboom.
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.
He held the position until July 2013, when he announced his abdication - citing ill health - and was replaced by his son, Philippe. He is said to receive an annual income of about €1m.
Shortly after, Ms Boël opened court proceedings to prove her paternity, provoking speculation the scandal had influenced the move.
Earlier this year, he refused to fulfil a 2018 court order to undergo DNA testing and launched an appeal.
A court had initially set a three-month limit for him to provide a saliva sample for testing - saying that without it, Ms Boël would be assumed to be his daughter and be eligible for any inheritance.
Belgium has a constitutional monarchy in which the king plays a largely ceremonial role.
Who is King Albert II ?
- Born in 1934 as second in line to the throne
- Married Italian Donna Paola Ruffo di Calabria in 1959
- The couple have two sons and a daughter
- Became king in August 1993 following the death of his older brother, King Baudouin
- With his constitutional role, stepped in during country's 2010-2011 political stalemate
- Served as monarch for two decades until his 2013 abdication
- Lost his immunity held as king after abdication