Madrid lost the opening tie 2-1 to their bitter rivals on home soil last week, taking the lead at the Bernabeu through Cristiano Ronaldo only for Barca to come from behind and win it on the night.

That defeat is seemingly the catalyst for rumours of discontent, with Spanish newspaper Marca claiming there are problems in the camp - both between factions of the squad and allegedly between coach Jose Mourinho and players Casillas and Sergio Ramos. Mourinho, meanwhile, was booed by some fans in last night's 4-1 Primera Division win against Athletic Bilbao, which kept the leaders five points clear.

Casillas has rubbished such talk, however, and was keen to put on a united front ahead of Wednesday's second leg in Catalonia.

"I don't have a bad relationship with any team-mate," the Spain captain told reporters. "We co-exist in training sessions, pre-match gatherings and games, like all other squads.

"I may get angry with someone one particular day, or someone may get angry at me, but it's not something to dwell on.

"There are no problems between the Spaniards and Portuguese on the squad. I find it quite absurd also because there happens to be Brazilian players amongst the so-called Portuguese group."

Turning his attention to matters on the pitch, he added: "We lost one game, the team didn't play well and we must now think about our next match in the Copa del Rey.

"It will obviously be complicated to win the tie, but I wouldn't rule it out. If Barcelona managed to win 2-1 over here why can't we do the same?

"More than ever we must be united. That's what matters."

Another finely-poised quarter-final this week sees Espanyol travel to third-tier Mirandes defending a 3-2 lead tomorrow.

Espanyol boss Mauricio Pochettino - whose side came from behind to win the first leg in the last five minutes - is happy to turn his attention to the cup following Saturday's 3-0 league win over Granada.

"We need to change our focus and think of the cup," he said.

"The players will rest as much as possible before the cup match because it will be another demanding game on Tuesday."

This week's other games see Athletic Bilbao take a 2-0 lead to Mallorca - who are selling tickets for just five euros to members in a bid to boost the support for their team - while Valencia will hope for a comfortable night at Levante as they hold a 4-1 advantage.