Harlequins flanker Chris Robshaw expects the revamped European Champions Cup to provide an even sterner test this season, reports BBC Sport.
Quins begin their continental campaign with a tough home clash with Castres on Friday night hoping for a better start than in last year's Heineken Cup - where they lost their opening match to Scarlets.
They failed to reach the quarter-finals, ending up in the European Challenge Cup, and Robshaw admits there can be no margin for error this time around.
He said: "One home defeat could be the end of your chances of reaching the quarter-finals. The tournament has gone up another notch this season with the switch to the Champions Cup format.
"You have to be on your mettle and properly prepared for every game in Europe. There are no margins for error and you have to be fit and ready."
The clash with Castres will provide Quins with a difficult start to their European campaign as the French side finished second in last season's Top 14 after defeat to Toulon in the final.
However, the visitors to Twickenham Stoop tomorrow have suffered a torrid opening to their domestic season, with six defeats in their first nine matches.
"We got caught cold in our opening game by the Scarlets last season and suffered as a result," the England captain added.
"What we've got to do on Friday night is be more precise and get our game going from the start."
Harlequins have themselves endured inconsistent early-season form, with three wins and three defeats in the Aviva Premiership so far.
They did however enjoy a comprehensive 52-0 victory over London Welsh in their last home game, and Robshaw is hoping the crowd can get behind his side in their quest for a win against Castres.
"We've had some great crowds at our home matches this season and the fact we will have the honour of kicking-off the Champions Cup just adds extra appeal to the occasion," Robshaw told the official Harlequins website.
"We always thrive off the support of the crowd and they will have an important part to play on Friday night against a tough team.
"Castres may not have made the greatest of starts in the Top 14 this season, but they still have a host of quality players.
"They will be looking at Europe as the chance to make a fresh start and we will certainly be paying them all the respect they deserve."
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