European air traffic controllers said there would be about 500 fewer flights taking to the skies on Monday, as Portuguese authorities said Lisbon airport re-opened a day before the pope flies in.

"According to the forecasts, during the afternoon areas of higher ash concentration could move in a north-easterly direction from the Atlantic into the Iberian Peninsula," Brussels-based Eurocontrol said in a statement.

The eruption of the Eyjafjoell volcano in Iceland, which caused travel chaos worldwide with airspaces closed over several European nations for a week last month, was again causing delays due to "significant re-routings" of transatlantic flights, Eurocontrol said.

"Areas of high ash concentration have dispersed overnight over continental Europe," it underlined, adding there were around 1,500 fewer flights than anticipated on Sunday.

Lisbon airport, which was closed on Sunday due to safety fears for engine tolerance amid the volcanic ash cloud, reopened Monday at 9:00 am (0800 GMT), civil aviation authorities there said.

The airport, Portugal's busiest, had originally been scheduled to remain shut until 1:00 pm but the volcanic ash cloud had moved away "more quickly than expected", a spokeswoman for civil aviation NAV told AFP.

Pope Benedict XVI is due to begin a four-day visit to Portugal by flying into Lisbon on Tuesday.