His likely successor, Thomas Jordan, is also a strong advocate of the bank's policy of capping the franc against the euro. The swift end to the scandal could even have repaired any damage to the central bank's reputation.

Hildebrand's announcement he would step down over a controversial currency trade made by his wife produced a knee-jerk 35-point blip up in the franc to trade close to 1.2100 francs per euro.

It was the highest level in almost four months and was based on speculation that the franc ceiling - imposed to keep the safe haven currency from getting too strong - may be at risk.

But the franc later pared its gains and last stood close to unchanged for the day around 1.2140 to the euro.

The franc has remained resolutely weaker than the 1.20 ceiling since the SNB shocked the markets and showed its determination to prevent further appreciation of the currency at all costs on September 6. The currency had risen to record highs in August as investors piled in as a protection against the euro zone crisis.

Following Hildebrand's quitting, however, market participants said a credible speculative attack on the 1.20 level was unlikely, with many traders having been badly bruised by the 10 percent rise in the euro/Swiss rate on September 6 alone.

"I don't think this will make any difference to SNB policy per se. It's not going to lead to a serious challenge of the Swiss franc ceiling or anything like that," said Simon Derrick, head of currency research at Bank of New York Mellon.

"It may be slightly counterintuitive, but it says you have a central bank you can trust and from that perspective it does make the Swiss franc marginally more attractive. But in the broader scheme of things we know the SNB is going to defend the ceiling and this just adds to the credibility."

Traders say a recent slow appreciation of the franc has been driven primarily by the broad-based sell-off in the euro at the start of 2012 but that a follow-through move through the 1.2000 threshold looked remote.

Following Hildebrand's resignation, the SNB said the franc cap policy would be pursued with utmost determination.

Latest Swiss data showed consumer prices continue to fall in December - in part because the strong currency makes imports cheaper - keeping up the risks to the Swiss economy and acting as justification for the SNB's policy on the franc.

With SNB Vice Chairman Jordan taking over in the interim and seen as the likely permanent successor to Hildebrand, markets look to be reassured over a smooth transition in leadership and a consistent exchange rate policy.

"Jordan has been as big a talker of the (euro/Swiss) floor over time as Hildebrand, so there's no reason for him to change policy," said Geoff Kendrick, currency strategist at Nomura.

(Additional reporting by Naomi Tajitsu. Editing by Jeremy Gaunt.)

By Neal Armstrong and Nia Williams