LONDON, Nov 10 Reuters If a reemerging risk premium in bonds is down to government debt sustainability worries, central banks may need to lobby their Treasuries that it39;s undermining their control of credit.

U.S. Federal Reserve officials are puzzling over why bond borrowing rates spiked lately even as Fed policy expectations have remained largely unchanged. Whether a resurfacing term premium39; now demanded to buy and hold longerterm bonds, is responsible is central to the conundrum.

If a sustained or even more volatile risk premium tightens or loosens credit beyond what39;s intended by the central bank, it clearly complicates its policy transmission to the wider economy at a critical juncture.

As most economists put the creeping premium down to nervousness around rising mounting public deficits, debts and bond sales and little prospect of them being reined in soon central banks may have to start an uncomfortable campaign of publicly warning their political masters.

Britain39;s brief budget and debt shock late last year and the way the Bank of England was forced to react was perhaps a taster.

But much hinges on degree to which investors are justified in demanding added compensation for fiscal jitters.

Former International Monetary Fund chief economist Olivier Blanchard this week framed the debt sustainability nerves around the relatively simple question of whether interest costs on the debt now exceed economic growth projections 39;r minus g39; in budget math…