UK airports have said they are running as usual, amid tightening security following US warnings over a "credible threat" of terrorism. Heathrow and Gatwick both said they were operating "as normal". Staff at Manchester are swabbing hand luggage. Number 10 said there was an "evolving threat" to the UK and other countries but people should travel as usual. US officials believe Islamist terrorists are developing bombs that could evade routine checks. The US is concerned that two branches of al-Qaeda are working together to try to design such a bomb.