While there has been much talk that Arctic trade routes will open up as northern ice melts, shipping companies and experts say using the fabled Northwest Passage through Canada's Arctic archipelago would be too difficult, too dangerous and totally impractical.

In theory, the idea is tempting -- the passage cuts the distance between Europe and the Far East to just 7,900 nautical miles, from 12,600 nautical miles through the Panama Canal.

Global warming means that the summer ice cover in Canada's Arctic is shrinking at such a rapid rate that experts predict the waters could be clear for at least part of the year within a few decades.

Yet few predict vessels will steam through the Passage in any great numbers.

The highly unpredictable nature of Arctic ice, a total lack of infrastructure, narrow channels, relatively shallow waters, increased insurance costs and the unwillingness of firms to take risks are all to blame.

"No one in the industry is really talking about the Northwest Passage being a serious alternative to the Panama Canal, even if it does open up at all," said Simon Bennett, secretary of the International Chamber of Shipping in London.

"There are navigational challenges, so many 'ifs' and 'buts' and the idea that you are going to take merchant ships with deep draughts through icy waters that are uncharted, really means that currently it is no match for the canal."

Even if the ice does melt in summer the season would be very brief, perhaps from late June to late September. Then the long dark winter starts drawing in and ice forms again.

And just because there is no ice in the passage one summer is no guarantee it will not return the next -- a factor which does little to assuage vessels seeking reliable routes.

Canarctic Shipping, by far the biggest operator in the Canadian Arctic, says the extremely high demand for cargo ships means owners do not need to look for short cuts.

"Ships are not like buses. They don't run on a schedule at 3:20 in the afternoon. They have to be positioned," said Tom Paterson, Canarctic's vice president of ship management.

"The big fear is that you get up there and you can't get through because you've got a block point ... and then you've risked the customer's cargo and have to reroute him back through Panama and he's not going to take that risk."

The Passage is in fact five different routes through dozens of rocky islands and narrow waterways. The southern route, the one the least likely to be affected by ice, is also one of the most challenging and not best suited to enormous ships that need plenty of water to maneuver.

Michael Gardiner, assistant commissioner of the Canadian Coast Guard for the Arctic, said there had only been 150 transits in the last 100 years, most by coast guard vessels.

"The Northwest Passage in its entirety has often been described at shipping conferences ... as a rock pile. It's very tricky navigation through most of it," he said.

And if a ship got into trouble the rescue effort would be massively complex and costly, said Bob Gorman of Enfotec, which provides ice navigation services in the Arctic.

"It's just not a route well traveled ... there are no tugboats nearby, there are no shipyards nearby, there are no repair facilities, there is no port of safe refuge. You are really out in the wilderness," he said.

Another challenge is the slowly rotating permanent ice cap at the top of the world, which is made up of diamond-hard multiyear ice that can easily tear holes in ships.

Chunks of this ice, which are hard to spot, occasionally make it into the passage and are set to do so more frequently as the Arctic warms and the giant cap slowly comes apart.

"We believe that the last ice in the Arctic to melt, whenever it is ... is likely going to be this ice," said John Falkingham of the Canadian Ice Service.

This puts the focus on other routes that might become ice free in summer, such as those over the North Pole and along the top of Russia.

Using the Northwest Passage is also hampered by bureaucracy. Canada claims control of the waters in the passage -- something the United States disputes -- and only allows ships to sail through if they are specially strengthened and follow a series of strict rules.

Most analysts do expect increased shipping going into and out of the Arctic to meet growing demand for the remote region's rich store of minerals.

But experts say that does not mean cargo vessels will be crowding through the passage as they seek a short cut.

"There's going to be a lot more people and lot more ships there but it's not going to be the Panama of the north," said Falkingham.

(additional reporting by Stefano Ambrogi in London)