Top drivers Sebastian Vettel and Fernando Alonso could be on the move, according to rumours in the Barcelona paddock.

Although leading the world championship for Ferrari, Vettel is rumoured to have inked a 'pre-agreement' to switch to Mercedes for 2018.

"I don't know," the German said in Barcelona. "Is [the rumour] coming from Italy? Maybe you should ask the Italians - they seem to know more than I do."

Most paddock insiders believe that if there is any truth to the rumour, it is probably just a negotiating ploy ahead of Vettel's contract talks.

"Seb has been first or second for the last four races," said his former Red Bull teammate Daniel Ricciardo. "I think in that case you don't want to leave."

More likely, perhaps, is that Vettel's teammate Kimi Raikkonen will leave, opening up a potential return to red for Fernando Alonso.

However, it is unclear whether Ferrari would pair Vettel with Alonso, who quit the Maranello team amid some acrimony three years ago.

"I'm signing my contract, I'm not signing other people's contract, so it's not my decision," said Vettel. "But I'm ready to race whoever comes along."

Spaniard Alonso's patience with struggling McLaren-Honda is almost certainly up, so when asked if he would consider returning to Ferrari he answered: "I'm open to anything."

However, a more likely contender for a 2018 Ferrari seat is Red Bull's Daniel Ricciardo, even though he is under contract.

"At the moment I have a contract with Red Bull, so unless some really strange things happen, my situation will not change," said the Australian.

Vettel leads the championship by 13 points ahead of Lewis Hamilton heading into Sunday's Spanish Grand Prix.