VW board members demanded Piech go after learning of secret plot

* Piech drew ire of key board members by reneging on CEO pledge

* Ouster of powerful chairman ends public row but leaves void

* Winterkorn must tackle strategic problems highlighted by Piech

* Some worry CEO will be beholden to unions who saved his job

By Andreas Cremer and Georgina Prodhan

BERLIN/FRANKFURT, April 27 (Reuters) - Ferdinand Piech, who resigned as chairman of Volkswagen over the weekend, sowed the seeds of his own demise by reneging on a deal to support CEO Martin Winterkorn and secretly plotting to oust him instead, according to sources close to the VW board.

When news leaked out last week that Piech had been lobbying family members behind the scenes to install Matthias Mueller, the chief executive of Porsche, at the helm of VW, the company's powerful works council and its home state of Lower Saxony - a top shareholder - decided they had had enough.

They demanded a meeting of senior board members - the second emergency VW summit in a little over a week.

"It was one of the straws that broke the camel's back," a source close to one of the VW board's labour representatives, told Reuters, referring to a doomed attempt by Piech to convince fellow family members to dump Winterkorn and install Mueller.

On Saturday, at an airport in Braunschweig, a half hour down the road from VW's imposing Wolfsburg headquarters, Piech was issued an ultimatum: resign or suffer the ignominy of being booted out in a vote of the board.

Neither Piech, Volkswagen nor senior board members would comment on the resignation, which was announced by VW and a committee of board leaders.

Piech's departure represents the end of an era for Volkswagen. For over two decades, the 78-year-old Austrian has ruled VW like his personal fiefdom, summarily ending the careers of executives he'd taken a dislike to and ramming through controversial strategic decisions through sheer force of will.

His shock ouster ends two weeks of public mudslinging at Volkswagen. But it also leaves a void at the top of Europe's largest carmaker and a host of questions about the future shape of the company.

Most urgent among them is who will replace Piech in the crucial role of chairman.

Winterkorn had long been seen as the natural successor, but can he slide into the post now, in the wake of his damaging row with Piech?

UNION POWER

Winterkorn has been freed from Piech's attacks but may also find himself more dependent than ever on the unions who saved his career. Berthold Huber, the former head of Germany's influential IG Metall trade union, takes over as acting chairman following Piech's departure.