The painful saga of Italy is by now well-known. The country is stuck in a depressionary debt trap. Trend growth is below zero. GDP is still 9pc below its pre-Lehman peak. Industrial output is back to levels reached thirty-five years ago.

The contours are worse than the 1930s. It is a lost decade turning into a second lost decade. No large developed country in modern times has ever suffered such a fate.

Italy is the victim of a vicious cycle of labour hysteresis as economic stagnation and weak productivity reinforce each other. Its exchange rate is overvalued by 20-30pc against Germany.

How easily we forget that Italy used to run a big trade surplus with Germany in the old days of the lira, and its real growth rate tracked German growth almost exactly with the help of devaluations. Each country was true to its political anthropology.

Italy cannot now deflate its way back to viability since this shrinks the underlying base of nominal GDP and automatically steepens the debt trajectory. It is impossible task for a country with a public debt ratio of 133pc of GDP, and is self-defeating in mechanical terms.

Reform is a beautiful word, but is almost meaningless at this juncture. There is no plausible way out for Italy within the current contractionary structure of monetary union. Only ECB bond purchases forever can keep the lid on this pressure cooker.

Yet it is patently obvious that QE is nearing political, legal, and technical limits. The ECB already faces a lapidary attack by Otmar Issing, its founding chief economist and a figure of towering authority in Germany.

He accused the bank of sliding down a "slippery slope", straying from genuine monetary policy and instead rescuing bankrupt states in violation of the treaties. "The no bailout clause is violated every day," he said.

The ECB has so far bought €1.4 trillion of bonds. Its balance sheet will reach 35pc of eurozone GDP by the end of the year at the current torrid pace, much higher than it ever reached in the US. Mr Issing said QE is nearing the point where the ECB may not be able to extricate itself without disastrous losses.

The inevitable taper battle is now raging within the ECB's governing council, with the two German members leading a swelling mutiny before the next meeting on December 8. Any suggestion that the programme will not be rolled over in full when it expires in March risks a financial storm. Italy will be the epicentre.

The context is fundamentally more dangerous than the events leading up  to the US taper tantrum in May 2013, when the Federal Reserve first began to talk tough.

It invites the perennial question whether Italy, Portugal, and perhaps others, can fund themselves at all in the capital markets, given that the eurozone has done almost nothing since the debt crisis of 2011-2012 to put monetary union on workable foundations.

There is still no fiscal union, no shared debt issuance, no banking union worth the name, and no expansionary New Deal to lift the economy off the reefs once and for all.  All it has done is to tighten surveillance, hoping somehow that it can muddle through by riding on world demand.

So Europe's taper tantrum - when it comes - ineluctably turns into a fresh stress test of monetary union itself. "The feeling once again is that the eurozone is not entirely safe," said David Owen from Jefferies.

David Fuller's view

The EU is unravelling at an accelerating pace.  This both saddens and disgusts me, because these are obvious, self-inflicted wounds, caused by an arrogant and corrupt Brussels bureaucracy.  I won’t vent further today, because I have so often been critical of the EU, not least since the Brexit vote.  Subscribers know my views.

However, here are 38 short Comments published by The Telegraph in response to Ambrose Evan’s Pritchard’s column.  I have copied them exactly as they appeared and deleted none.  They are varied and mostly interesting on the EU.  A few fret about AEP’s erudite and multilingual vocabulary.  I enjoy it and welcome the addition of new words or phrases.