By Martin Shankleman
Employment correspondent, BBC News
Tesco has opened stores under the Fresh & Easy name in the US
Tesco's treatment of workers abroad has come under attack from the international trade union, UNI.
In a series of reports the UNI union, which is based in Switzerland, makes allegations about the firm's behaviour in Thailand, South Korea and the US.
It accuses Tesco of firing workers who wanted to form a union and making employees work 24-hour shifts.
Tesco, the UK's largest supermarket, said the allegations were a "travesty" and "misrepresent the truth".
UNI claims that Tesco:
• required employees in Thailand to work 24-hour shifts which was "shocking", fired employees who applied to form a trade union and was guilty of systematic and deliberate breaches of health and safety
• forced employees in South Korea to work up to 20 hours a week on unpaid overtime and made the leaders of a company union resign for settling a long-running labour dispute
• employed only part-time workers in the US, thereby preventing employees from earning a living wage, and rejected a legitimate recognition request to form a union in one California store.
At a news conference in London, Phil Bowyer, deputy general secretary of UNI, urged Tesco to begin discussions to address the problems highlighted: "We ask Tesco to take these reports seriously and take remedial action."
He said that previous attempts to raise concerns with the company's British headquarters had been rebuffed, with Tesco claiming the issues should be answered by management in the local countries.
He called on Tesco to admit whether or not it wanted to do business with unions. "If you don't want to deal with trade unions, say it. We then have ways of dealing with you," he said.
Tesco denied it was anti-union and claimed all its staff were free to join trade unions.
It attacked the reports as politically motivated and said they had been produced by a union trying to recruit more members.
"The reports are a travesty and misrepresent the truth," said a Tesco spokesman in the UK.
"UNI are using a standard tack of going to far off places, producing reports which are very difficult for people in the UK to check. We do check these matters and can tell you that the allegations are untrue."
It added it had productive relationships with unions in a number of countries, including Usdaw in the UK and Solidarity in Poland.
The union also released a copy of a letter Barack Obama wrote to Tesco's chief executive Sir Terry Leahy, pleading with the company to begin talks with the United Food and Commercial Workers Union (UFCW) in the US.
In the letter, written last June while he was campaigning for the presidency, Mr Obama criticised the company, which trades in the US as Fresh & Easy, for not meeting with union officials.
"I again urge you to reconsider your policy of non-engagement in the United States and advise your executives at Fresh & Easy to meet with the UFCW and other community groups at the earliest opportunity," he wrote.
Tesco said Sir Terry replied that it was the company's intention to run a good business in the US , and he pledged to observe the guidelines relating to union membership set down in US legislation.