The move will reopen Heathrow Airport in London, one of the busiest airports in the world, which has not had a commercial flight land since Thursday afternoon.

The announcement came with more than two dozen British Airways flights already in the air, bound for London. They took off hoping there would be a window of opportunity to land the planes at Heathrow or Gatwick airports, a company representative told CNN.

The 25 planes took off from the West Coast of the United States, Africa, India and other locations in Asia. A 26th plane that had hoped to reach London was sent to Brussels, Belgium, instead, British Airways said.

Asked if Heathrow was prepared to receive the inbound flights, airport operator BAA told CNN it would follow instructions from air traffic controllers.

Half of all Europe's flights were back in the air Tuesday.

Eruptions from the Eyjafjallajokull volcano are weakening, authorities in Iceland reported, but volcanic ash already in the air continued to blow toward Britain.

About 14,000 flights were expected to operate in European airspace Tuesday -- half of scheduled air traffic, according to Eurocontrol, the intergovernmental body that manages European air travel. Two-thirds of scheduled flights were canceled Monday, and nearly 80 percent did not fly over the weekend.

Flights on Monday were restricted to those above 20,000 feet, Eurocontrol said. Lower airspace is closed or severely restricted across Europe, it said.

Planes were back in the air in France, Germany and Italy. Around 75 percent of scheduled flights were expected from France's two main airports: Charles de Gaulle and Orly, French Minister for Ecology Jean-Louis Borloo told French radio network RTL.

Germany's air authority, DFS, allowed flights to take off and land in German airports at low altitude under visual flight rules to reduce the risk of damage to airline instruments. The country's main carrier, Lufthansa, said it hoped to operate as many as 330 flights, including some long-haul flights, Tuesday.

Italy has reopened its airspace nationwide, the Italian aviation authority ENAC announced.

While airports were open in Spain, the ban on flying in British airspace left thousands of Britons trapped there. A British Navy vessel, HMS Albion, was dispatched to pick up 300 civilians from Santander, Spain, along with hundreds of British military personnel who were returning from Afghanistan, Britain's Ministry of Defense said.

Some countries opened their airspace to travel, while keeping airports closed.

Norway opened part of its airspace near Bergen briefly Tuesday and predicted Gardermoen, just north of Oslo, would be open all day, the country's air traffic control service AVINOR said.

Meanwhile, Finland confirmed that all Finnish airports would remain closed until 9 a.m. (2 a.m. ET) Wednesday.

British Airways issued a statement Tuesday apologizing to its customers for the continued disruption to its flight schedule. It said it was working with other airlines to provide data about the ash to authorities.

A British Airways test Sunday found no damage to the aircraft at a variety of altitudes, the airline said Monday.

"The analysis we have done so far, alongside that from other airlines' trial flights, provides fresh evidence that the current blanket restrictions on airspace are unnecessary," said Willie Walsh, British Airways' chief executive.

Finland's Ministry of Defense, however, said in a statement Monday that Finnish Air Force F-18 Hornet fighters that conducted test flights over northern Finland on Thursday showed effects from volcanic material.

Meteorologist Derrick Ryall of the Met Office in London said there is no "defined safe limit" for volcanic ash.

"No one has said what a safe limit for aircraft is, so therefore it's very difficult for these judgments to be made," he said.

We really need to work out what the minimum ash concentration is ... for safe flight. We need to establish what that is, because if we don't, we can expect to see repeats of this sort of disruption," said Simon Day, a volcanologist at University College London.

Since the volcanic eruption worsened last week, sending an ash cloud into the skies, airlines have been losing at least $200 million a day, according to the International Air Transport Association, the trade group representing airlines.

Airports had lost close to 136 million euros ($184 million) as of Sunday, said Olivier Jankovec, director general of Airports Council International, Europe.

More than 6.8 million passengers had been affected, Jankovec said in a statement, adding that the effect is worse than after the September 11, 2001, terrorist attacks on the United States.

Thousands of people have been living in airports.