"The law firms are trying to get ahead of the game," said Stephen Ross, head of civil fraud group litigation and arbitration department at Withers in London. "They're seeing if they can sign up enough people to bring proceedings."
Class action suits—legal action brought by a large number of people—are easier to get off the ground in the U.S. rather than the U.K., according to Ross. "It's easier to get people on board … you have to get people to opt in here, but in some U.S. states it's more of an opt out."
Tesco declined to comment on the matter.
"This is your classic class action ambulance chasers," Tony Williams, founder of London law consultancy Jomati Consultants, told CNBC in a telephone interview. "When they say an 'investigation', they're trying to get a few people together to do a class action and litigate."
Williams added: "The minute companies have a profit warning or anything else, you'll see these king of adverts trying to get a class action going … most of them peter out but sometimes they score."
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Andrew Johnston, a specialist in company law and corporate governance at Sheffield University, doubted the law firms would be successful if they litigated.
"There isn't a very strong incentive for minority shareholders to litigate," Johnston told CNBC in a telephone interview. "They won't be able to prove fraud, and negligence won't give them the remedy they want."
Earlier this year, a U.S. judge ruled that a group of shareholders in BP—a FTSE 100-listed company like Tesco—could pursue a class action lawsuit against the oil giant. The shareholders say the oil giant mislead them by understating the severity of the 2010 Gulf of Mexico oil spill.
The Tesco scandal comes as the supermarket continues to struggle with falling sales and declining market share. British shoppers are increasingly opting for discount rivals such as Aldi and Lidl. Sales at Tesco fell 4.5 percent in the 12 weeks to September 15 compared to the same period in 2013, according to research provider Kantar World.
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