Britain's two busiest airports have closed as a volcanic ash cloud drifts further south, threatening major disruption to many thousands of people.<br/><br/>A no-fly zone imposed by the Civil Aviation Authority ceased Heathrow, Gatwick and London City airports. <br/><br/>Flights are also grounded in parts of Scotland and Northern Ireland. <br/>Manchester, Liverpool and Leeds Bradford airports have re-opened after restrictions in the north were lifted. <br/><br/>Birmingham, Norwich and East Midlands airports are also open again, after suspending flights yesterday. <br/>