UPDATE 3-S.Africa regulator cast doubt on Vodacom listing

* Regulator's move spooks investors, rand weakens

* Regulator says Telkom stake sale needs approval

* Rand drops 3 percent

* Markets see Vodacom as key hurdle for Zuma govt

* Vodacom says listing on track, seeking legal council

* (Adds analyst's comments, rand reaction)

By Gugulakhe Lourie

JOHANNESBURG, May 15 (Reuters) - South African regulators demanded on Friday that Telkom, the country's biggest telecoms operator, get approval to sell a stake in mobile firm Vodacom scaring investors and raising doubt about the biggest bourse listing in months.

News of the about-turn by telecoms regulator ICASA pushed the rand currency as much as three percent lower and analysts said the surprise decision only three days ahead of the listing is a key test for President Jacob Zuma's new government.