* Drivers harden stance after union meeting
* Alonso wary of Silverstone Pirelli test
* Boycott would not be a first for the sport
By Mark Meadows
NUERBURGRING, Germany, July 4 (Reuters) - Formula One drivers announced on Thursday they will withdraw from this weekend's German Grand Prix if the tyre blow-outs which marred Sunday's British race occur again.
During news conferences, drivers sounded confident that tyre provider Pirelli had sorted out the problem by introducing new rubbers for the Nuerburgring race. However, following a meeting of the drivers' union, the tone hardened.
"The drivers of the Grand Prix Drivers' Association (GPDA) wish to express their deepest concerns about the events that took place at Silverstone," a statement said.
"We trust that the changes made to the tyres will have the desired results and that similar problems will not occur during the German GP weekend.
"We are ready to drive our cars to the limit, as we always do, and as it is expected by our teams, sponsors and fans.
"However, the drivers have decided that, if similar problems should manifest themselves during the German GP, we shall immediately withdraw from the event, as this avoidable problem with the tyres endangers again the lives of drivers, marshals and fans."
Five drivers suffered blowouts at Silverstone on Sunday and a piece of tyre narrowly missed the head of Fernando Alonso of Ferrari. Felipe Massa was badly hurt after being struck on the helmet by a loose spring during a race in 2009.
Massa talked of a possible boycott immediately after Sunday's race, won by Nico Rosberg of Mercedes.
A boycott would not be a first for the sport, which has experienced several walkouts by teams and drivers in its often turbulent history.
In 2005, seven teams pulled out of the US Grand Prix in Indianapolis because of tyre safety fears.
In 1985, the French Ligier and Renault teams shunned the South African Grand Prix - the penultimate race of the season - amid mounting international pressure against the apartheid system of racial segregation.
In 1982, only 14 cars started the San Marino Grand Prix at Imola after a boycott by teams - led by current commercial supremo Bernie Ecclestone - fighting the governing body in a political battle dubbed the 'FISA-FOCA war' within the sport.
That same year the drivers organised a strike before the South African race at Kyalami over restricting clauses in their superlicence agreements. The race went ahead after a compromise was reached.
In 1976 the Austrian Grand Prix was run without Ferrari, whose Austrian driver Niki Lauda was in hospital after a fiery crash at the Nuerburgring and who were incensed at McLaren's James Hunt having his Spanish Grand Prix win reinstated.
ALONSO FEARS REPEAT
After the intervention of the governing FIA, Pirelli has brought upgraded rear tyres to the Nuerburgring for this year's German Grand Prix with an inner belt made of the synthetic fibre Kevlar rather than steel.
Spaniard Alonso, second in the standings behind Red Bull's triple world champion Sebastian Vettel, said earlier on Thursday he was reluctant to take part in a test at Silverstone later this month because he feared a repeat of the blow-outs.
Formula One's governing body responded to the tyre blowouts by deciding on Monday to change the rules and allow race drivers to test revised Pirelli tyres, instead of using young drivers.
"I don't intend to go," Alonso told reporters in the Ferrari motorhome. "It's not a very safe thing racing on the same track with the same tyres. I don't have the feeling I want to go. But if the team wants me to go..."
Before the GPDA statement, double champion Alonso had put his faith in Pirelli to make this race a safer one, even if cooler rear tyres could benefit Mercedes and Red Bull based on this season's analysis.
"Pirelli made some changes so we trust them," he said. "This is the third or fourth change they make in the season already, some were hurting us, some were helping us. I think performance today is the second priority.
"The most important thing is to make it for dinner at home on Sunday. I was lucky... It (the piece of tyre) could have hit my helmet. It would have been like a bullet."
"MIX THINGS UP"
Vettel said before the GPDA announcement: "Like we said last weekend, it has to be safe. It was not what we want, not satisfactory. It's good we have a new tyre here."
Red Bull team mate Mark Webber, who is swapping F1 for the equally dangerous pursuit of Le Mans endurance driving at the end of the season, rejected suggestions that kerbs at Silverstone had ruptured the tyres.
"All of a sudden in the race there were landmines everywhere which no one really predicted," Webber said.
"In the end everyone left there in one piece... apart from the tyres. Pirelli will learn from it, they have to. There's no issues with the kerbs because the track was fine the last five years. I'm a bit more confident they've got more data."
Pirelli has put some of the blame for the Silverstone blowouts on the teams for the way they managed its tyres. It conducted a "secret" test with Mercedes in Barcelona in May for which the German team were punished by the FIA.
Mercedes have been ruled out of the Silverstone tyre trial this month.