The Constitutional Court will decide today on the legality of Prime Minister Prayut Chan-o-cha living in an army residence since his retirement from the military.
A ruling against the prime minister would mean the end of his premiership, and also end the term of his current cabinet.
Pheu Thai accused the prime minister of a conflict of interest by continuing to live in the army residence in Bangkok.
It said Gen Prayut, who is also the defence minister, should have left the building when he retired as army chief in 2014, and that by staying on free of charge, he had broken army regulations.
The prime minister and his family reside in the house in question located at the 1st Infantry Regiment, King's Guard, on Vibhavadi Rangsit Road in Bangkok.
After leading a coup in 2014, Gen Prayut retired as army commander-in-chief on Sept 30, 2014.
He remained prime minister and was returned to office following last year's general election.
The army has explained that the house the prime minister lives in was re-designated as a "visitor's house" in 2012. The army has also cited security as the reason why Gen Prayut needs to live inside the army compound.
