UPDATE 1-Telefonica bid for GVT a blow to Vivendi-sources

* Telefonica's GVT bid a "major blow" to Vivendi - sources

* Vivendi may cash in gains from accord with GVT holders

* Terms of accord not disclosed by sources (Updates with funds that recently boosted stakes in GVT)

By Guillermo Parra-Bernal

SAO PAULO, Nov 6 (Reuters) - Telefonica's decision to sweeten its offer for Brazilian telecom rival GVT dealt a "major blow" to Vivendi's plans to acquire the carrier, sources close to the transaction told Reuters on Friday, suggesting the French media giant may walk away from the deal.

But an existing agreement between GVT's controlling shareholders, the Swarth Group and Global Village Telecom, to tender at least 20 percent of their outstanding shares to Vivendi (VIV.PA) could present the French company with a "good exit strategy," said the sources, who asked not to be named because a transaction has yet to be completed.

Telefonica (TEF.MC) raised its offer for GVT (GVTT3.SA) by 5.2 percent on Nov. 4, seeking to trump a potential Vivendi counteroffer and seal a deal before the end of the month.

Under the terms of the counteroffer, the bid was upped to 50.50 reais a share, valuing the deal at about $3.9 billion, from 48 reais a share, or $3.7 billion, previously.

Vivendi would have to offer a minimum 53.02 reais a share, or 5 percent premium to the initial bid, to muscle out Telefonica. A decision to withdraw from the deal would be in line with Vivendi Chief Executive Jean-Bernard Levy's long-standing policies of only buying assets that do not risk the company's investment-grade debt ratings and of paying high dividends.

The sources declined to elaborate on the terms of the accord with GVT's controlling shareholders.

A Vivendi counteroffer would risk sparking a bidding war for GVT, something Levy has always pledged to avoid. Vivendi's $3 billion friendly approach had yet to be formalized.

Shares of GVT, based in the southern Brazilian city of Curitiba, rallied after Telefonica's Wednesday announcement.

GVT's common stock has advanced 3.3 percent since Wednesday, hitting an all-time-high of 53.35 reais on Friday.

Analysts, including Valder Nogueira of Itau Securities, have recently brought up the accord as a possible roadblock to successful completion of the deal by Telefonica.

Vivendi's relationship with Swarth and Global Village is "very good," the sources said.

Telesp (TLPP4.SA), Telefonica's unit in Brazil, is determined to win swift regulatory approval for the deal before a scheduled tender offer on Nov. 19.