BANGKOK (AP) — Authorities in Thailand have escalated their effort to collect taxes they say are due from ousted Prime Minister Thaksin Shinawatra, posting a bill for 17.6 billion baht ($503 million) at the house where he lived before fleeing into exile in 2008 to avoid a prison term for conflict of interest.

The Revenue Department acted Tuesday as part of a plan announced earlier to avoid a March 31 legal deadline on collecting the money. Presenting a formal tax assessment covering Thaksin’s sale in 2006 of shares worth 73.3 billion baht ($1.88 billion) in his telecommunications company to a Singapore state holding company is supposed to keep alive the government’s claim.

One of Thaksin’s lawyers, Noppadon Pattama, said his team will challenge the bill. Thaksin’s supporters say the action is politically motivated.