June 28 (Reuters) - Talks between U.S. and Britain regarding a travel corridor are increasingly unlikely to reach a conclusion by the end of July, the Financial Times reported on Monday, citing officials. The officials added the rise in cases of the Delta variant in Britain, the complexities of the U.S. political system and uncertainty over the status of the AstraZeneca vaccine in the United States, where it is not yet approved, were set to extend the talks into August and even September, the newspaper said. Officials in London hoped they would have the outline of an agreement to reopen travel between the United States and Britain by July 4, the newspaper added. A British government spokesperson told Reuters both countries established a working group "to help relaunch UK-U.S. travel as soon as possible." "Discussions between the working group are ongoing to ensure the UK and U.S. closely share thinking and expertise on international travel policy going forward," the spokesperson added. Earlier in June, U.S. President Joe Biden and British Prime Minister Boris Johnson discussed opening a transatlantic travel corridor at their bilateral meeting, but no announcement was imminent, British Foreign Secretary Dominic Raab said. read more On Monday, Spain announced that will demand a negative COVID-19 test or proof of vaccination from British tourists coming to the country after letting them in freely for more than a month. read more Portugal also announced British visitors must quarantine for 14 days from Monday if they are not fully vaccinated against COVID-19. read more (This story was refiled to remove UK government"s no comment line.)