FRANKFURT, Jan 14 (Reuters) - If Greece were to leave the euro zone and start using a national currency, its sovereign credit rating would fall to junk status, rating agency Standard & Poor's said on Thursday.
S&P Managing Director Moritz Kraemer, writing in a newspaper column, said he did not think any country would quit the 16-country currency bloc and the costs of such a move would exceed the benefits.
But if Greece, which was downgraded by all three major ratings agencies late last year, were to leave the bloc, its sovereign debt would likely be downgraded even further.
S&P currently rates Greece BBB+, still within investment grade.
"According to estimates by Standard & Poor's, the Greek rating could fall after a (euro zone) exit by approximately four steps and thus into speculative grade (non-investment grade, BB+ or lower)," Kraemer said in a guest column in German daily Boersen-Zeitung.
If Spain, Portugal or Italy were to leave, their ratings "could fall by two to three notches. And these simulations do not even take into account possible serious or political crises," he added.
If a country leaving the euro zone were to re-denominate its public debt in the new national currency, he said, that would be even worse, amounting to country default.
While European Central Bank officials have said there is no chance of a euro zone break-up, some analysts have toyed with the idea in recent weeks as investors have demanded an increasingly high yield premium to hold government bonds from countries with a less solid fiscal track record compared with Germany and its benchmark Bunds.
The cost of credit default swaps insuring Greece's debt rose to 328 basis points on Wednesday, or $328,000 per year for five years to insure $10 million in debt, from 281 basis points on Tuesday, according to CMA DataVision. (Reporting by Sakari Suoninen; Editing by Ruth Pitchford)