By forcing the EU to explicitly detail support for Greece, markets are showing they will no longer tolerate fuzzy "implied guarantees" to underwrite the creditworthiness of troubled borrowers.

Since the formation of the euro, Greece, Spain and other fringe euro economies have been borrowing at lower interest rates on the market assumption that they were effectively backed up by richer core eurozone states such as Germany.

But for months, EU and Greek policymakers have been refusing to publicly discuss what support might actually be there, fearing that to do so would take the pressure off Greece itself to push through painful reforms and cuts.

Repeatedly pushed both on and off the record by journalists, they simply clammed up, saying such matters should never be discussed.

Meanwhile, Greece came under greater and greater market pressure, pushing its borrowing costs to unsustainable levels, in part because investors struggled to price in accurately the prospect of a bailout.

But investors continued to treat Greece more leniently than non-euro troubled countries, believing the colossal potential fallout from Greek default or currency breakup would ultimately force the EU's hand.

The cost of insuring Greek debt in the default swaps market was less than half that for Ukraine, where it still costs investors around $1 million a year to protect $10 million of five-year debt.

On Thursday, after days of hints, EU President Herman Van Rompuy said eurozone officials had reached a deal to help Greece - although an EU diplomat said the methodology was still being worked out.

The spread between Greek debt and benchmark German bunds narrowed to the tightest in two weeks on the news.

The greater the support, the riskier bunds will be priced against US, Japanese or other non-euro zone debt as markets reprice the risk from Greece to Germany.

Similar market alarm over Ireland in 2009 was eased when the then-German Finance Minister Peer Steinbrueck brought calm by saying that all the eurozone countries would help "if it came to a serious situation" - seen as an effective guarantee.

With the current Greek crisis, Mr Steinbrueck was out of office and other policymakers offered much more mixed signals. In any case, this time markets wanted more.