European airspace is now partially open with a small number of flights resuming from northern Europe on the sixth day of the "volcano blockade". New violent volcanic activity, however, and a new volcanic cloud drifting south and east over the UK, have kept British airports mostly closed.

This means that there are still more than 400 000 Britons stranded by the Icelandic volcano eruption, while about 40 000 American citizens desperate to fly to the US are stuck in the UK.

Sofia airport is operating with flights to and from Vienna, Istanbul, Moscow, Athens and Bucharest.

Previously scheduled flights from London to Sofia, Easy Jet flight EZY8973, Brussels flight by Wizz Air W6622, Munich flights of Lufthansa and United, LH3434 and UA9246 respectively, and an Easy Jet flight EZY2671 inbound from Milan, have all been cancelled.

Departures from Sofia to London, Easyjet Ezy8974 and a Wizz Air flight W6649 to Venice have also been cancelled.

With new volcanic cloud descending over the UK, it is still unclear when British authorities will allow their airports to open, which will cause considerable stress for those still stranded in Bulgaria wishing to leave for the UK or vice-versa.

Initially, authorities were hoping that Heathrow airport would open from 9pm Bulgarian time on April 20. UK air traffic control body, Nats, said the flight ban would be lifted over Scotland, Northern Ireland and the north of England, but those hopes also seem to have been dashed.

"This demonstrates the dynamic and rapidly changing conditions in which we are working," said a statement from Nats, quoted by the BBC.

According to the UK Daily Mail, weather experts were subjected to "hostile attacks" from air carriers, who were accusing them of spreading panic and publishing false information.

Newcastle International Airport said it planned to re-open for flights to and from Aberdeen and the Isle of Man at 9am Bulgarian time.

Easyjet said it would operate a "very limited service" from late afternoon on April 20.

Meanwhile, Manchester Airport, which initially contemplated opening at 11am Bulgarian time, has decided against, on advice from Nats.

So in many cases, the situation remains largely as it was the previous five days. Passengers are advised to contact their airline before travelling to the airports, particularly if those airports are in the UK.