The People's Democratic Reform Committee (PDRC) rallies led by former Democrat MP Suthep Thaugsuban have so far caused more than 70 billion baht in economic damage, said Pheu Thai deputy spokesman Anusorn Iamsa-ard.
If the PDRC really shuts down the capital as threatened on Jan 13, foreign investors will no longer have confidence in Thailand, Mr Anusorn said on Saturday. Foreign capital inflows will be reduced substantially, and prices of shares on the Stock Exchange of Thailand would sink, worsening the ongoing economic slowdown, he added.
The SET lost 5.7% of its value on the first two trading days of this year, the worst start to a year for the local stock market since 1988.
Mr Anusorn called on businessmen and others who have been supporting the anti-government rallies to think twice before joining the planned "Bangkok shutdown".
He warned that it could take more than 10 years to heal the economic damage if the event really occurs.
In a related development, Caretaker Deputy Prime Minister Surapong Towichakchaikul warned the public that the planned Jan 13 shutdown would be illegal.
Speaking in an address televised by the Television Pool, he said disruption on the scale threatened by Mr Suthep would be a violation of the law and would create difficulties for citizens and business operators.
The government had directed all state agencies to come up with backup plans for the possible capital shutdown, he added.
Legal actions would be taken against all PDRC co-leaders who violated the law, said Mr Surapong, who also heads the Centre for the Administration of Peace and Order (Capo).
He urged people not to support the PDRC rallies as they had already caused severe damage to the country.
Mr Surapong also called on people to turn out to exercise their voting rights in the Feb 2 general election in order to bring about peace and allow the country to move forward under a democratic system.
Mr Suthep announced on Friday night that the protesters' main rally stage at Democracy Monument would be shut down on Jan 13.
Rally stages would be set up across the capital at 20 major intersections to disrupt the work of government officials and paralyse Bangkok to pressure caretaker Prime Minister Yingluck Shinawatra to step down, he said.