Barack Obama, the Democratic presidential hopeful, is urging Sir Terry Leahy, Tesco's chief executive, to engage with trade unions in the US over workers rights at the supermarket giants Fresh & Easy chain.

In an unprecedented call, the US presidential candidate has called on Sir Terry to reconsider Tesco's policy of non-engagement with the unions, just two days before the supermarket giant's annual general meeting with shareholders.

In the letter, seen by The Times, the Democratic party candidate says: "It is in your interest to ensure that the communities and the leaders of workers in the industry are heard and that their concerns are heard.

"I am aware of Tescos reputation in Britain as a partner of unions. I would hope that you would bring those values to your work in America.

"Ensuring that workers are able to exercise their right to organise and work in safe environments were an important element of my campaign for the Democratic nomination as they will be in my campaign for the Presidency.

"I 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 (the United Food & Commercial Workers Union) and other community groups at the earliest opportunity."

The letter is the second that Mr Obama has sent to Tesco, after writing to Tim Mason, Fresh & Easy boss and the supermarket chain's US president, pressing him to work with local community leaders to strike a so-called community-benefits agreement - a written pledge of promises about the rewards its opening would bring to local areas.

Mr Obama's latest intervention comes three weeks after the UFCW said it would be seeking the support of the presidential hopeful after trying in vain to force a meeting with Tesco over the past two years.

It is furious that Tesco refuses to recognise the union at Fresh & Easy.

Emily Stewart, UFCW campaign director, argues that the union only wants the same constructive relationship that Tesco has with Usdaw, the shop workers union, in the UK.

The UFCW launched a Two Faces of Tesco campaign to highlights its greviances, and has been backed by Jon Cruddas, the Labour MP who stood for the deputy leadership of the party two years ago.

He said Tescos stance was undermining the reputation of British companies around the world.

Earlier this month, Tesco said: "The UFCW do not reflect the views of Fresh & Easy staff who are overwhelmingly in favour of the pay and benefits they receive."