Spyker Cars pressed ahead with efforts to cut a deal for Saab with General Motors, with talk of possible backing from a Dutch billionaire fanning the Swedish carmaker's faint hopes of an eleventh-hour reprieve.

Shares in the Dutch luxury carmaker rose sharply on Tuesday after local media reported the bid, modified after GM said on Friday it would wind Saab down despite Spyker's ongoing interest, had won support from financier Marcel Boekhoorn.

But in comments to a newspaper Boekhoorn denied he was involved, and the market remained skeptical that Russia-backed Spyker -- which extended its new offer to buy Saab shortly before it was set to expire on Monday -- would clinch a deal.

Sweden's IF Metall union held meetings with Spyker on Tuesday and said the company's business plan was a copy of an old plan by Koenigsegg, the Swedish luxury carmaker which dropped its bid for Saab last month.

Spyker has conducted stress tests on that plan, Stefan Lofven, chairman of the union, told Reuters after meeting with Spyker Chief Executive Victor Muller.

Spyker is anxious for this to become a reality, Lofven said. From what we can see, this seems serious. They have put in a lot of time and money into this.

Dutch paper De Telegraaf quoted unnamed sources as saying Boekhoorn, who bought a 5.7 percent stake in Spyker in 2007, was involved. But the financier told the daily: It's all not true. It's been made up.

Still, Spyker shares closed up 6.8 percent at 2.19 euros, short of Monday's four-month high of 2.30 euros and with trading volumes of just 98,373 shares encouraging price volatility.

The new offer came as a surprise, and now there is the (report) of ... Boekhoorn who slipped in through the back door. That gives the shares a boost, said Keijser Capital trader Michael Jabroer.

(But) I think this is going to be capital-destructive ... Spyker may have to raise capital from the market and whether it will be fully used for Saab remains to be seen. Perhaps it will be another toy for Muller.

Swedish daily Svenska Dagbladet, citing unidentified sources, said on Monday the ownership structure backing the Spyker bid had been altered to placate worries at GM, and that Russian parties were no longer involved.

Russian banking tycoon Vladimir Antonov holds a stake of almost 30 percent in Spyker, and Russia is keen to obtain Western technology to re-energize its car industry.

Russian state-controlled Sberbank and Canada's Magna tried to buy a stake in GM's Opel unit until GM decided to keep it last month.

SPYKER'S MULLER HOPEFUL

A spokeswoman for Boekhoorn's company Boekhoorn M&A and Saab both declined comment. A spokesman for GM Europe repeated that GM would look at all offers for Saab but declined to give details.

Speaking from Stockholm, Spyker's Muller said he was still in talks about buying Saab and was still hopeful but declined to comment on the Boekhoorn report.

The Boekhoorn spokeswoman declined to say if the financier, whose wealth is estimated at 1.3 billion euros by Dutch magazine Quote, was still a shareholder in Spyker.

He sprang to prominence when he bought a stake in Dutch telecoms group Telfort in 2003 and made more than 300 million euros two years later when the firm was sold to KPN .

Abandoning the 60-year-old Swedish auto brand would eliminate 3,400 jobs in Sweden and hit 1,100 Saab dealers, but GM raised hopes on Sunday when it said it would evaluate several new expressions of interest for Saab.

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