They love their buzzwords, in the bustling, multi-multilingual, smart-suited bureaucratic Nirvana of the European Union capital.

But for years "Brexit" was a swearword beyond the pale. No one wanted to hear, no one would speculate, and basically nobody believed that Britain could possibly leave the EU.

Now, you hear it whispered. And a horrible realisation is growing: it may be too late to stop.

Meet the chief prophet of doom. Fabian Zuleeg, chief executive of the European Policy Centre, is super-smart and mainlines the information flow inside the Brussels bubble. On Brexit he's matter-of-fact and utterly gloomy. He has a clearly reasoned and logically devastating argument that Brexit is a possible, even probable result of Britain's current "crisis" in its relationship with Europe.

"If you had mentioned a possible Brexit to people five years ago they would have said it would never happen," he said, in a briefing last week to a group of foreign journalists, including Fairfax. "There was a taboo in this country about Brexit, you wouldn't actually talk about things like that.

"That has changed. If you're talking to people now Brexit is on the agenda. It is also the first time that I have heard some people say at the European level maybe it's better if it happens.

"That isn't the agreed position … But even the fact that some people are thinking about it in this way is quite a change in the tone of the debate here."

Brexit is very different, in meaning and consequences, to Grexit.

Grexit is taken to mean Greece leaving the Eurozone in order to default on its loans, devalue the resurrected drachma, and renegotiate with borrowers on the basis of a currency it could afford and control.

There would be financial chaos: a massive flight of capital as everyone hoarded euros ahead of the switch, a government default followed by corporate and bank defaults.

This is a nightmare that analysts (and the market) refuse to believe will happen, despite last weekend's triumph in Greece's elections of the left-wing Syriza, which campaigned on an end to Brussels-imposed austerity. Greek PM Alexis Tsipras told his cabinet this week the country would not default on its debts. European countries are already suggesting they could extend repayment deadlines.

Britain, of course, never joined the Euro, so a Brexit means it leaves the European Union. The consequences are up for debate. Foreign companies using the City of London as a bridge to Europe may relocate to Frankfurt. Europe's global political power would diminish, and its ability to strike favourable trade deals. Britain would find itself disempowered as its former European partners debated Russia, Syria, climate change and other threats to security and prosperity (goes the theory;  UKIP's Nigel Farage claims the opposite, that Britain's voice would be louder with the Brussels filter removed).

But, Zuleeg argues, none of this matters much. His argument goes like this.

Step one: will there be a referendum in Britain? The Conservative Party is the only major party to have promised one, and on current polls would not win a majority in May's general election.

However UKIP, the explicitly anti-Europe party, is buoyant in the polls and might prop up a minority Conservative government, with a referendum as a condition of support.

UKIP is also hurting Labour in its heartland, making it harder for the party to explicitly reject a referendum. Labour may find a referendum "one of the easiest things to concede", Zuleeg says.

Step two: What will be the outcome of a referendum?

"The optimists say pragmatism will prevail," says Zuleeg. "The people of Britain are not that stupid, they will look on the impact on economic realities and decide that… it is far too costly to be outside the European Union.

"This is a plausible scenario. But it is not one I believe in."

A lot of people will campaign strongly against the EU. But he doesn't see anyone to campaign strongly for it.

The Conservative Party is now "pragmatic Eurosceptics and outright Eurosceptics" – and will be even more so after the election. Labour is torn, by fear of UKIP and desire to watch the Conservatives squirm and split. Business, while generally pro-EU, will have some voices arguing (says Zuleeg) "if you get out of Europe you can have a deregulated financial system and be the global centre of finance, you can then do things without all those pesky laws on bonuses etc".

And in the media, most of the opinion-shifting, top-selling tabloids tell an endless, scathing narrative about EU red tape and diktat.

Zuleeg has watched a new, growing bloc of English nationalism. In this group's minds, the EU means uncontrolled immigration. "It's quite frightening. If you look at the general attitudes towards migration [in Britain] it's intensely negative and I think this is going to drive a lot of this agenda."

Polls suggest a majority will vote for the EU if the prime minister, after the election, can negotiate reform in Europe.

The trouble is, no-one can begin to describe reforms that would be both sufficient and possible.

Some Europeans are proposing "nice protocols which reinforce the role of national parliaments, which talk about preventing abuse of the freedom of movement and labour".

"A Conservative PM coming back with such a package would be shot down by his own eurosceptics," Zuleeg said.

But Europe won't agree to anything bigger. "It opens up too many other nightmares" – a queue of 27 other countries making their own demands – and anyway there just isn't the time.

"We are in grave danger that the expectations being built up about what might change are far higher than anyone can deliver," Zuleeg says. Free movement of labour is fundamental to the EU's treaty bedrock. And no-one will give Britain an "out" on collective decisions.

"That is actually the benefit of the EU, exactly what makes it work, this is exactly why we are managing not to kill each other. We are sitting down together and having these negotiations and coming to some conclusions that of course will not be ideal for everybody but in the end it is a collective decision.

"That's what makes the EU, that's what makes the EU in any way relevant in the global environment. If we give that up then we might as well, basically, go back to medieval times when everybody sits in their castle and hopes the world doesn't come to them."

If Brexit is not to happen, then Europe needs to send this message quickly, clearly, and in a way that is somehow palatable to a country that, at the moment, doesn't want to listen.