European air traffic agency Eurocontrol said on Monday areas of higher ash concentration could move from over the Atlantic Ocean back toward the Iberian peninsula, threatening fresh airspace closures in Portugal and Spain.
Europe has been dogged for weeks by repeated shutdowns of air traffic since an erupting volcano under the Eyjafjallajokull glacier in Iceland started spewing ash in April.
Hundreds of flights were canceled over the weekend after the shifting cloud sparked fresh fears of engine-clogging ash.
Italy and Germany reopened their airspace on Sunday, but restrictions were left in place in parts of Austria, Britain Portugal and Spain.
Spanish airports operator AENA said that from 1400 GMT until at least 1800 GMT restrictions to stop planes flying between 20,000-35,000ft would reduce capacity at Madrid, Barcelona and Seville airports by 25 percent.
At Madrid, landings would be cut to 38 from 46, AENA said in a statement, adding it was working with the Ministry of Defense to use military spaces for commercial use during the restrictions.
The biggest closure so far lasted for almost a week from April 15, causing about 100,000 flight cancelations, stranding millions of passengers and costing airlines more than $1.7 billion in lost revenue.
Volcanic ash is abrasive and can strip off aerodynamic surfaces and paralyze aircraft engines. It can also damage aircraft electronics and windshields.
The most recent closures led to fresh criticism of how air traffic authorities are handling the situation. Germany's Lufthansa and Air Berlin called for authorities to gather data on ash particles rather than just relying on computer models.
Europe's airlines are now starting to tally up the ash cloud's damage to their business in terms of passenger numbers.
Air France-KLM said it lost nearly a quarter of its European passenger traffic in April due to airspace closures from the volcanic ash cloud, driving total traffic down 16 percent.
Irish airline Aer Lingus said it carried over a quarter fewer passengers. British airports handled 23 percent fewer passengers in April, owner Ferrovial said.
Fraport, operator of Frankfurt airport in Germany, was due to publish April traffic figures as well as first-quarter results on Tuesday.
The hit to April passenger traffic came just as airlines around the world have started to recover from a severe drop in traffic that reached its severest point in March 2009.
According to industry body International Air Transport Association, the world's airlines lost about $9.4 billion last year as customers curbed spending during the recession.
They stand to lose another $2.8 billion this year, excluding any impact of the volcanic ash cloud.
Air France-KLM reiterated that every day on which it has to completely suspend its flights lowers its net operating result by 35 million euros.
Lufthansa, due to publish traffic figures on Tuesday, has said it lost almost 200 million euros due to volcanic ash in April.
"The snafu over the weekend will likely cost Lufthansa a few more million euros and be visible when traffic figures for May are released," LBBW analyst Per-Ola Hellgren said.
Airline shares rose as a rescue package to tackle the euro zone debt crisis lifted markets across Europe.
