Barack Obama has again called on Tesco, the UK supermarket group, to let unions represent workers at its new US chain. The move comes as the Democratic candidate seeks to reinforce his labour support ahead of November’s presidential elections.

Mr Obama urged Tesco to meet the UFCW grocery workers’ union to discuss “the principles of partnership” at its Fresh & Easy stores in a letter to Sir Terry Leahy, chief executive, on the eve of the retailer’s annual meeting.