Red Bull would have "severe reservations" about signing McLaren's Lewis Hamilton to partner Formula One world champion Sebastian Vettel, team boss Christian Horner said on Tuesday.

Speculation about Hamilton's future has continued unabated since last month's Canadian Grand Prix, where the 2008 world champion publicly sought out Horner in the Red Bull paddock after qualifying.

While the 26-year-old's McLaren contract has another season to run, the most recent reports have focused on a possible break clause that might allow him to leave before that.

"We have no knowledge of what Lewis' contractual situation is with McLaren. It's obviously an issue between him and the team," Horner said at a media picnic lunch at the championship-leading team's factory.

"We're focused very much on our own drivers, and on our own performance.

"On paper, a Vettel-Hamilton lineup would be hugely attractive, but you have to look at the reality at how these things work," said Horner.

"History dictates that two world-class drivers haven't always been the best pairing if you think of Piquet-Mansell, or Senna and Prost or even Alonso and Hamilton."

Famously fiery

Hamilton (pictured) raced alongside Spain's double world champion Fernando Alonso at McLaren in 2007 in a famously fiery pairing that saw their relationship start amicably and end in frosty silence.

Red Bull's current lineup is German championship leader Vettel and Mark Webber, with the Australian out of contract at the end of the year and yet to decide his future.

"We are hugely happy to have Sebastian committed to the team for the long term, and with Mark we're very pleased with the job he is currently doing," said Horner.

"Our focus at present is on those two drivers rather than looking to change anything."

Asked by Reuters why Red Bull - who have won six of the first eight races of the year with Vettel - should worry about the drivers getting on when their focus was also on generating publicity for their energy drink, Horner replied: "At the end of the day it's about winning. It's not solely down to Red Bull to provide the entertainment. We do enough of that.

"If you look at any great team, whether it be a football team, a hockey team or across the disciplines, it's about the unit working as a unit, and not individuals," he said.

"That's what we've managed to achieve here, and that's what's hugely important. No individual is bigger than the team. I think it would be difficult to envisage a driver of Sebastian's caliber and Lewis under the same roof."

Hamilton has just one win this season with McLaren and has made more headlines of late for controversial collisions and run-ins with the race stewards.