* Sees Nabucco costing up to 26 billion euros
* Lack of gas supply, buyers a problem for Nabucco
* Threatens lawsuit against French group over tax payment (Adds detail)
By Tom Bergin and Michael Roddy
LONDON, Oct 24 Hungary's Minister For National Development raised doubts about the viability of the Nabucco gas pipeline on Monday and said he might sue a French utility which he believed had not paid enough tax.
Tamás Fellegi, who has authority for energy matters, said Nabucco was in doubt because it had not yet secured a supply of gas or customers and because the cost of building it was soaring.
"I see plenty of problems surrounding Nabucco. Number One: it is fully in the fog what the ultimate price of Nabucco will be," he told a briefing for reporters in London.
"We started out with around 7 billion euros for the full project, now the very optimistic scenario runs around 24-26 billion, almost four times higher than the original one," he added.
A spokesman for the Nabucco consortium, which is led by Austria's OMV and includes Germany's RWE , Hungary's MOL , Turkey's Botas, BEH of Bulgaria and Romania's Transgaz <ROTGN.BX, said the minister's estimate was "random".
However, he added the consortium's own 7.9 billion euros estimate was under review.
Earlier this month, European Union energy commissioner Guenther Oettinger said the project could cost up to 14 billion euros.
Fellegi added that he was considering suing a French utility over tax payments, accusing the group, which he declined to name, of taking out unreasonably large management fees from its Hungarian unit, thus reducing taxable profits.
Paris-based groups EDF and GDF Suez operate in Hungary.
A spokeswoman for EDF in Hungary could not immediately comment and GDF Suez did not respond to requests for comment. (Additional reporting by Gergely Szakacs in Budapest; Editing by David Cowell)