Deep in the forest of Gribskov, some of the leaves are starting to flush with their autumnal colours. It makes the stand of blighted trees all the more obvious.

Seven or eight 20m-high sticks, stripped of their greenery, are all that is left of a group of ash trees. "It's very ugly and very sad," said Ditte Christina Olrik, a scientist with the Danish government's Nature Agency. "The fungus is very, very small, like pinheads, on the leaf stalks. When the leaves die and fall, the wind carries them on to the next tree."

The little killer that has affected almost all of Denmark's ash trees, and has stealthily destroyed ash in Germany, Poland, Norway, Sweden and Austria has a big name – Hymenoscyphus pseudoalbidus, an anamorph of Chalara fraxinea. For those who enjoy a walk in the woods without a Latin degree, it's ash dieback.

The Danes, along with people in other European countries who have watched this fungus destroy their trees with a force not seen since the Dutch elm disease epidemic, have been urging Britain to ban imports of ash saplings to keep forests here quarantined.

Last week the UK government launched a consultation that ends on 26 October and could lead to an interim import ban by November. But what the environment minister announced as "timely" action might in fact be far too late.

In February, the fungus was found in a batch of trees sent from a Dutch nursery to Buckinghamshire. Between June and September it was confirmed in nurseries in Yorkshire, Surrey and Cambridgeshire, at a Forestry Commission Scotland woodland near Kilmacolm, and in ash trees planted in a Leicester car park. Conservationists hope it has not reached the 80 million ash trees in the wider British countryside, outside of new plantings.

In Denmark 90% of their ash has been affected, their third most common tree species after oak and beech and a crucial export for the timber industry, and expect that to rise.

In the state-owned forest in Zealand, north of Copenhagen, forest pathologist Iben Margrete Thomsen says their hope is that around 1% of ash in Denmark may prove resistant and survive.

Passing a stand of balding trees, she points out one that still has a normal leaf cover – "but, of course, we don't know if that is truly a resistant tree or one that will be affected very soon. In the end, though, only the few ash trees which are not susceptible to the fungus will be left.

"At first it was only the younger trees, and then we started to see it in the older trees too. Now I can see it everywhere. The fungus itself doesn't immediately kill the tree, but if you think of it acting a little like the HIV virus it weakens the immune system and allows some other disease to come in. It is the secondary disease that will finish the tree off.

"We have not seen anything on this scale since Dutch elm disease. Fungus is meant to kill trees, otherwise a tree would never die, but this escalation is something we have not seen. The UK and Ireland may well be, within the next 10 years, the only remaining places in Europe with ash. If they act.

"The rest of Europe wants the UK to act. They are saying 'get a ban' because they have seen what this disease can do. But whether or not it's too late?" She shrugged. "It may be a case of shutting the stable door when the horse has bolted. Time will tell."

Outside of the forests, ash was used in towns and cities too, to replace many of the urban trees Denmark lost to Dutch elm disease.

On Sonde Boulevard in the capital, Copenhagen, Lars Christensen is among a team of horticulturalists responsible for the city's landscaping. They, too, have seen ash dieback and here on this street have come up with their response: variation. Along the street that runs through a residential area are planted almost a dozen different species of young trees.

"It's about rethinking the two rows of identical trees in cities. It's simple, but surprisingly difficult to convince people that we cannot be reliant on one tree species, no matter how much people like them," he said.

City trees have a tough time. Root restriction, pollution, salt and dog urine mean they have a life expectancy that is half that of a fellow forest tree. But constantly cutting down and replanting diseased trees is costly and last year Copenhagen brought in a 10% rule. "Every tree we plant we have to check how many of that species is in an area already," explained Christensen, showing a great list of statistics for the city's areas. "If, say, we already have 10% or more of ash then we have to plant lime or willow or whatever."

But even if the city's busy residents will be protected from the sudden loss of their trees, the ash will be badly missed.

In the national forest at Gribskov, Olrik points out an ash that has been cut down, how the beautiful light-coloured wood that makes ash so popular for furniture and other uses is discoloured and blotched.

The tree trunk has an alien look, covered with unnatural-looking, thin green sprouts coming through the bark. "The tree panics when the leaves die, so to try to stay alive, it pushes out lots of these things. It's one of the signs.

"Seventy years of growth and now good for firewood only," sighs Olrik. "We cut this tree to show your UK Forestry Commission when they came here to have a look last week. They were very shocked.

"It's shocking. We are used to seeing ash dieback now, but it's still hard to think we are losing the ash tree. Not just economically, not just because it supports a unique and complex ecosystem that we will also lose, but it's such a traditional tree for Denmark; the tree of life it's called.

"You see many houses that will have a huge ash tree outside. It was supposed to protect the home from fire. Every Danish child knows a story about the ash tree, it is very important in mythology, and their death represents chaos.

"So now they are dying, some people are wondering, what does it mean? What is the future?"

• This article was amended on Thursday 8 November. The headline and main text said that 90% of trees in Denmark had been killed. This is incorrect and has been changed to "affected".