By Jake Maxwell Watts and Nopparat Chaichalearmmongkol 

Thai companies targeted by antigovernment protesters for their perceived links to Prime Minister Yingluck Shinawatra and her family have lost more than $2 billion in market value in less than a week.

The call from protest leaders for a boycott to pressure the embattled administration has caused share prices to plummet and even swept up Advanced Info Service, Thailand's largest telecoms provider, in a subscriber exodus despite the company having severed ties with the Shinawatras in 2006.

Shares of SC Asset PCL, a property developer where the prime minister previously served as chairwoman that is still majority owned by Shinawatra family members, has fallen more than 9% on the Thai stock exchange since the boycott was called last Wednesday. Family-run handset distributor M-Link Asia Corp.'s shares are down nearly 11%.

The benchmark SET index lost 1.7% in the same period, underperforming its regional peers as investor confidence has eroded from the political standoff.

Ms. Yingluck has vowed not to resign despite being targeted in a campaign that has roiled the capital since November and left 21 people dead. She now heads a caretaker government pending a resolution to elections held Feb. 2 that she had hoped would renew her mandate but were sufficiently disrupted by the opposition that a new parliament cannot be seated.

Brokers say most sales of the stocks are coming from local retail investors. "There is a feeling that the protests will have some impact on future earnings," said Itphong Saengtubtim, head of research at KGI Securities. Many of the protesters come from the "middle-income class and up so these types of people invest in the stock market."

The boycott battle has been fought on both sides of the political divide. People in northern Thailand, a mostly rural region that has benefited strongly from the populist policies of Ms. Yingluck and her brother, former Prime Minister Thaksin Shinawatra, began boycotting the iconic Singha beer brand after the heiress to brewer Boon Rawd began appearing at the front of antigovernment rallies. The company, which is privately owned, didn't respond to requests for comment.

Uncertainty has already prompted several companies to delay investment plans, including Chinese auto maker Great Wall Motor Co., which postponed its plans to build a sport-utility vehicle plant Tuesday, citing political unrest as a reason.

Among Thai companies affected by the boycott is telecommunications provider Advanced Info Service, which was founded by Mr. Thaksin and sold to Singapore's state-owned investment company Temasek Holdings Ltd. before he was ousted as premier in a coup in 2006. Mr. Thaksin lives mostly in Dubai to avoid serving jail time for a corruption conviction that he says was politically motivated.

AIS's share price has fallen 5.5% since the protesters called for the boycott.

In an emailed response to inquiries, Temasek said it doesn't comment on share price movements of any of its portfolio companies.

Rival telecoms operators True Corp. and Total Access Communication PCL. have offered discounts to entice subscribers thinking about switching. Several thousand have switched from AIS, Thailand's telecom regulator has said, including 2,000 on Monday alone--almost three times the company's usual rate. This marks a rare net loss for AIS, which usually has around 1,200 customers join and 700 leave on a typical day.

Chuleekorn Jarunchaisong, a 44-year-old state enterprise employee, switched service from AIS after a decade as a subscriber. "Switching providers is like taking an action to let (Mr. Thaksin) know my stance toward him," he said. "I think it is fair not to support his businesses."

AIS has texted subscribers that it was no longer associated with its founders. A spokesperson told The Wall Street Journal that the company is worried about being caught up in the political battles but is confident in the quality of its service.

Thailand's other telecoms providers aren't the only ones seeing opportunity in the AIS selloff. "I notice that foreign investors have bought the stock in the past two days," Maybank Kim Eng's head of research Sukit Udomsirikul said.

Aberdeen Asset Management, which manages around $1.1 billion of assets in Thailand and owns AIS stock, is content to sit on the sidelines for now absent a significant impact to earnings.

"Obviously, there is going to be negative short-term sentiment toward those stocks but being in a very dominant position in its business we think long-term [AIS] is still going to be quite good," said Adithep Vanabriksha, chief investment officer at Aberdeen's Thailand office.

Companies linked to Mr. Thaksin have been targeted before. When he sold his stake in Shin Corp., the parent of AIS, tax-free to Temasek in 2006 for $1.9 billion, a boycott was threatened.

Shin Corp. lost 38% of its value and AIS shed 28% in 2006, the year Mr. Thaksin was toppled. Both stocks more than recovered those losses.

Write to Nopparat Chaichalearmmongkol at nopparat.chaichalearmmongkol@wsj.com

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