Harlequins will have to go to Castres and win next week if they are to progress in the Heineken Cup.

And the way in which the visitors convincingly overturned the south London side on Sunday suggests that will be a tall order indeed.

Scotland international Gregor Townsend was inspirational for Castres, kicking five penalties and two conversions.

Fellow Scot Shaun Longstaff and Kiwi Norman Berryman also got on the scoreboard.

Quins had started well with a Dan Luger try after just five minutes.

England centre Will Greenwood fed his international team-mate and the winger side-stepped his way through the Castres defence to touch down.

Mark Mapletoft, standing in for the injured Paul Burke, converted but Castres soon bounced back with two penalties from Townsend.

Mapletoft added another penalty but that proved to be Harlequins' last score for more than an hour as the visitors put their foot down and took charge.

After 13 minutes Longstaff scored a superb try on the right wing after centre Eric Artiguste and Berryman had combined to split the Quins defence.

Townsend missed the conversion but made amends with two further penalties to put the visitors 17-10 ahead.

Then it was Berryman's turn to cross the line after Townsend had brilliantly switched the play wide.

Townsend successfully converted this time to put his side 24-10 ahead at the break.

Winger Longstaff scored his second try of the afternoon before Berryman matched his effort, giving Castres a massive 26-point lead.

Forward Steve White-Cooper salvaged some pride for the home side as he sneaked around the blindside of a ruck and ran 45 metres upfield for a converted try with just eight minutes remaining.

But a fifth Towsend penalty for Castres then sealed their triumph.

Harlequins: Tries: Luger, White-Cooper. Cons: Mapletoft 2. Pens: Mapletoft.

Castres: Tries: Sarramea 2, Berryman 2. Cons: Townsend 2. Pens: Townsend 5.

Att: 5,512

Att: 3,700.

Harlequins: O'Neill, Moore, Greenwood, Greenstock, Luger, Mapletoft, Bemand, Leonard, Wood, Dawson, Morgan, White-Cooper, Sanderson, Tamarua, Diprose.
Replacements: Fuga, Olver, Davison, Winters, M. Powell, Burrows, Gollings.

Castres: Mola, Sarramea, Artiguste, Berryman, Longstaff, Townsend, Albouy, Toussaint, Ibanez, Moyle, Laluque, Spanghero, Costes, Lassissi, C. Fernandez-Lobbe.
Replacements: R. Vigneaux, Dima, Tsabadze, Taussac, Froment, Gargallo, Teulet.

Referee: David McHugh (Ireland).