A gargantuan ash cloud from an Icelandic volcano is drifting over Britain, disrupting air traffic, prompting changes in President Obama’s travel plans and bringing back memories of the massive flight cancellations that stranded millions of passengers last year.

The ash cloud from the Grimsvotn volcano stretches from Greenland to Russia. But because of new policies governing air travel, the cloud appears unlikely to spark a repeat of the global transit nightmares of April 2010, when the eruption of a different volcano, Eyjafjallajokull, triggered the largest shutdown of European airspace since World War II.

Still, the return of the volcanic menace has Europe on edge — particularly its airlines, which lost billions of euros during last year’s cancellations.

Britain’s meteorological agency, known as the Met Office, declared a “temporary danger area” over Scotland until 1 p.m. local time (8 a.m. in Washington) Tuesday, leading British Airways to suspend all morning flights between London and Scotland. Dutch carrier KLM, no-frills airlines EasyJet and Ryan Air and numerous other regional airlines canceled flights to and from Scotland and northern England.

About 500 flights were canceled across northern England and Scotland on Tuesday, according to Eurocontrol, the European aviation authority. It warned of a “strong possibility” that the ash cloud could affect parts of Denmark, southern Norway and southwest Sweden by Wednesday. Given the new procedures for calculating flight risks, however, the agency said that the impact on flights “is expected to be relatively low.”

Since the chaos of last year — when flights were grounded in overwhelming numbers, turning much of Europe into a no-fly zone — officials have revised their understanding of when ash particles pose a danger.

They now say assumptions that volcanic clouds holding low amounts of ash were extremely hazardous are faulty. Revamped models have determined that flight conditions become hazardous only when they involve medium- or high-density portions of the ash cloud.

The new parameters mean a much more limited danger zone for flights. In Britain, rules established last year allow airlines to make their own decisions about whether to fly in skies containing volcanic ash, although they must apply for licenses from civil aviation authorities to do so.

A spokeswoman for Britain’s Civil Aviation Authority, who declined to be named, citing agency policy, described the broad no-fly policies of last year as “inflexible.”

“This time, there are more working models,” she said. “The CAA is not saying you cannot fly; instead, we’re looking at the science of how you can fly in a safe manner.”

“If aircraft are not flying, it’s their decision,” she said, referring to individual airlines. Though no permit is required for flying in low-density ash, airlines must apply for permission to fly in medium- or high-density ash clouds, and must include a sign-off from aircraft and engine manufacturers.

The spokeswoman said that some airlines have already made “safety cases” for flying in medium-density ash, but no airline has requested permission for flying in high-density ash.

The busy international air hubs in London were unlikely to be affected by high-particle ash clouds, and much of Europe would be spared, too, said Barry Gromett, a forecaster at the Met Office.

But he cautioned that the outlook could worsen, depending on the duration of the volcanic eruption and prevailing winds.

The threat of treacherous conditions on Monday led Obama to cut short his visit to Ireland and make haste to London for the second leg of his visit.

Correspondent Karla Adam contributed to this report.