The eruption of an Icelandic volcano, which had slowed in recent days, strengthened on Monday, spewing a new cloud of ash that officials said was heading toward the United Kingdom - possibly posing a renewed threat to air travel.

The statement came after millions of travelers stranded on both sides of the Atlantic were given a glimpse of hope earlier in the day, with officials announcing a plan to partially reopen European skies to air traffic.

About 8,700 flights have taken place in European airspace Monday, said Eurocontrol, the European Organisation for the Safety of Air Navigation. A normal Monday would see about 28,000 flights, it has said.

Check the latest developments regarding problems for air travel

Commercial European flights will be severely disrupted as long as some levels of ash are detectable in the air, the UK Civil Aviation Authority (CAA) told CNN on Monday.

Despite growing pressure from air travel groups such as the International Air Transport Association (IATA) and major airlines like British Airways, the CAA said that all current jet engine manufacturers specify zero levels of atmospheric ash for safe flying.

See an explainer on how aircraft engines react to ash clouds

The CAA added that unless jet engine manufacturers changed their operating specifications, something it added was highly unlikely in the short-to-medium term, the restrictions will continue to apply.

Travelers will need even more patience when airliners are finally cleared to fly, reports CNN.com’s Agnes Pawlowski. The huge backlog of passengers combined with flights that are already full means they may not be going home anytime soon.

The ash cloud is casting a shadow over the nascent economic recovery in Europe as the cancellation of flights in key markets entered its fifth day, reports CNN’s Kevin Voigt.

The Eurozone - the 16 European nations united under the euro currency - is in the midst of a shaky recovery. After shrinking 4 percent last year, the Eurozone is expected to grow only 1 percent this year, according to a forecast by Ernst & Young released last Friday.

"The key is how long this eruption and the disruption last," said Frederic Neumann, an HSBC economist in Hong Kong. "If it's just a couple weeks, from a macroeconomic standpoint it's just a blip on the radar ... if it lasts for months and months, then it's a different story."

CNNMoney’s Aaron Smith reports that the flight cancellations are costing U.S.-based airlines tens of millions of dollars per day, according to an analysis from an airline expert.

The chaos, however, has benefited some groups and companies, reports CNN’s Thair Shaikh.

Some of the visible winners of the volcanic ash flight chaos include coach companies, ferry operators, car hire firms and national train operators, including Eurostar, which has laid on extra trains in an attempt to cope with the explosion in demand.

The volcanic eruption in Iceland is having an adverse impact on international sport with next weekend's Japanese MotoGP the latest casualty.