Asked this week whether he believed Moscow would again cut off gas to Ukraine and therefore to much of Europe this winter, Andris Piebalgs, the European energy commissioner, said he thought it a "realistic probability." Some 80% of Russian gas supplies to the European Union pass through Ukraine and gas to Ukraine can't be interrupted without also stopping flows further west.

But not everyone views this prospect with the same dread as East European householders. One suspects senior executives at some major European energy companies would be secretly delighted if Russia stopped gas deliveries. The reason: they could buy the gas far cheaper on the world market than they are buying it from Russia.

In fact, this has the makings of a serious longer term challenge for OAO Gazprom, the state Russian gas monopoly that supplies 40% of the European Union's gas. The gas giant has short-term worries too. Gas demand has collapsed in Europe with the economic slowdown. Gazprom's sales to big European buyers such as E.On, BASF, Eni and GDF Suez have fallen to minimum contract levels, probably about 80% of contracted amounts, energy specialists say.

Unlike last winter, gas storage is full -- though there remain EU countries such as Bulgaria with almost no storage. The buyers would love to take even less from Gazprom but they can't because, under their so-called take or pay contracts, they would have to pay fines if they do.

Contract details are confidential but prices of Gazprom contracts are linked to oil prices with a lag of several months. Energy experts reckon that contract prices are probably around $10 per million British thermal units -- and because oil prices started climbing earlier this year, Russian gas prices will soon follow. Meanwhile, you can pick up liquefied natural gas on the world market for around $4 per million Btu.

The main reason for this gaping differential is a rapid expansion in output from North America. Developers have exploited new technologies to get gas out of the ground that was previously locked in. The growth of this so-called unconventional gas output has encouraged the prediction from the Paris-based International Energy Agency this month of "an acute glut of gas supply in the next few years."

"Gas suppliers to Europe and Asia-Pacific will come under increasing pressure to modify their pricing terms and cut prices to stimulate demand," it forecast in its World Energy Outlook.

But while at the moment Western Europe can bring this international gas into its networks, Eastern Europe can't. This divide, an affront to the idea of a single European market, is a "new iron curtain" that splits East from West, Pierre Noël of the European Council on Foreign Relations told a Brussels audience this week.

West of the divide, gas flows in two directions. East of it, the legacy of the past means that the pipelines allow gas to flow only from East to West, bringing gas from Russia but not allowing gas from the West. Last year's interruptions of supply have prompted renewed efforts to increase storage in Eastern Europe -- Poland's for example is due to double by 2012 -- and also to permit a reverse-flow capability.

One giant obstacle, however, is that Gazprom owns some of the key pipelines bringing Russia gas to the West. Why would it allow the building of so-called interconnectors that will allow competitors' gas to enter Eastern Europe from the West?

How long this new iron curtain will last depends in part on how soon the EU can get its act together to push forward a single market in an environment where, as one senior EU adviser said this week, the bloc "doesn't address this with a strategic view."

A reason for that, said Charles Grant of the Centre for European Reform, is that "national energy champions" -- by which he means the likes of Eni and E.On -- have persuaded national governments to keep the European markets segregated.

Gazprom can help accelerate the progress to a single market by continuing to cut off gas supplies to paying customers. For lots of reasons, therefore, another bitter Russia-Ukraine gas dispute is the last thing Gazprom, as a business, needs.

Gazprom, though, is more than a business. It is also an arm of Russian foreign policy.

Ukraine, whose economy has been among the hardest hit in the world by the global economic crisis, faces a presidential election Jan. 17. If Ukraine can't pay for its gas, the Kremlin may not be able to resist teaching President Viktor Yushchenko a lesson by cutting off his country's gas supplies.

Vladimir Putin met Ukraine's prime minister, Yulia Tymoshenko, on Thursday to discuss their energy relations. On Wednesday, Ms. Tymoshenko sought to ease concerns about Ukraine's reliability as a transit nation, saying it hasn't missed a payment in the 10 months since signing a deal with Russia.

But watch out for this around Dec. 7 and Jan. 7, when Ukraine's payments are due. The people of Eastern Europe and European energy executives certainly will be.

Write to Stephen Fidler at stephen.fidler@wsj.com