16-6 up at half-time, Wasps had looked to be in control, but a try by prop Aksventi Giorgadze just after the break made life uncomfortable, and Gordon Ross delivered a sublime performance with the boot to keep Castres in contention to the last.

Despite their lowly standing in the French league, Castres were dominant for long spells - particularly in the line-outs - and Wasps were lucky to avoid a defeat that could have all but ended their European campaign at the first time of asking.

They had edged in front right on the half-time whistle with a Paul Sackey try, and a conversion added to three penalties from Jeremy Staunton.

But their only second half score was a late penalty from Mark van Gisbergen as Castres responded with two penalties from Gordon Ross, who converted a second-half try from Georgian prop Giorgadze.

And there was also worrying news for England after Josh Lewsey - earmarked for the full-back berth against New Zealand in a fortnight's time - was forced off with a hamstring injury.

Wasps needed to hit the ground running in a tough group that will be shaped by their two games against Perpignan - starting away from home in six days time - before the Catalans come to Adams Park in January.

The 2004 European champions were riding high in the Guinness Premiership and unchanged from the side that beat Newcastle 35-15 a week earlier, meaning Lewsey was forced to play at outside centre again.

But on a dreadful weekend for English clubs in the Heineken Cup - following defeats for Sale, Gloucester and London Irish - nothing could be taken for granted in a constant downpour that forced a string of handling errors from both sides.

Although Castres included Scotland fly-half Ross and former England wing Phil Christophers in their line-up, it was their formidable pack that provided the biggest obstacle.

Staunton kicked penalties after six, 17 and 33 minutes to edge his side in front, with Ross replying twice, and Wasps' best efforts were counter-attacks from long range until they established a toehold in the Castres 22.

The try finally came with the last move of the half after a dozen phases when Sackey accelerated in from his wing onto a short pass from Eoin Reddan and through a gaping hole in the defence around a ruck 10 metres out, and Staunton followed up by converting from in front of the posts.

Castres responded by kicking for the corners and set up a scoring position when Wasps were forced to concede a five-metre scrum five minutes into the second half.

The first drive for the line was repelled but Georgian prop Giorgadze forced his way through Lawrence Dallaglio's tackle to score, with Ross converting from wide on the right.

And for those who had watched Wasps suffer at Edinburgh in this competition last season the alarm bells were ringing as they were forced to defend deep inside their own half for long spells.

Lewsey was forced off after aggravating a hamstring injury and his struggling side made a tactical switch when Alex King replaced Staunton at fly-half after 55 minutes.

But they survived more near-misses on the hour when first Pablo Canavosio went close and then Ross's cross-kick just eluded diving wing Laloaoa Milford.

Wasps finally broke out in the 70th minute when Tom Voyce chased a long clearance to force a five-metre scrum.

Castres flooded offside after Dallaglio picked up and van Gisbergen kicked the resulting penalty.

His side hung on for the final 10 minutes for the win and the performance will have worried no-one watching in Perpignan, who are already a bonus point up on their rivals after running in four tries against Treviso.

The scorers:

For Wasps:
Try: Sackey
Con: Staunton
Pens: Staunton 3, Van Gisbergen

For Castres:
Try: Giorgadze
Con: Ross
Pens: Ross 2

London Wasps: 15 Mark Van Gisbergen, 14 Paul Sackey, 13 Josh Lewsey, 12 Fraser Waters, 11 Tom Voyce, 10 Jeremy Staunton, 9 Eoin Reddan, 8 Lawrence Dallaglio (c), 7 Tom Rees, 6 Dan Leo, 5 Tom Palmer, 4 Simon Shaw, 3 Phil Vickery, 2 Raphaël Ibañez, 1 Tim Payne.
Replacements: 16 Joe Ward, 17 Peter Bracken, 18 George Skivington, 19 James Haskell, 20 Simon Amor, 21 Alex King, 22 Ayoola Erinle.

Castres Olympique: 15 Romain Teulet, 14 Laloaoa Milford, 13 Phil Christophers, 12 Rafael Carballo, 11 Bradley Fleming, 10 Gordon Ross, 9 Christophe Laussucq, 8 Florian Faure, 7 Patrick Tabacco, 6 Alexandre Bias, 5 Lionel Nallet (capt), 4 Pascal Pape, 3 Jeremy Castex, 2 Romain Terrain, 1 Akvensti Giorgadze.
Replacements: 16 David Roumieu, 17 Daniel Saayman, 18 Karim Ghezal, 19 Florent Fourcade, 20 Pablo Canavosio, 21 Jonathan Wisniewski, 22 Stuart Moffat.
 
Referee: Alain Rolland