Christian Horner has warned Red Bull's rivals that the defending champions remain as hungry as ever going into the 2013 F1 season.

For the past three seasons, the Milton Keynes-based outfit has secured both championship titles, with Sebastian Vettel starting the new campaign as the man to beat in the race for the crown.

Speaking to Sky Sports as pre-season testing resumed in Barcelona, Horner insisted that the champions aren't resting on their laurels heading into 2013 and remained fully motivated to succeed in a campaign he expects to be close.

"All the signs at the moment are that it is very tight and all the usual culprits look like they will be there," he said. "It is so difficult to read testing times as everyone is running to their own agendas, but what you can see is that by the time we get to Melbourne it is going to be very, very close.

"[However] The bottom line is we just need to keep doing what we are doing. We put the pressure on ourselves to perform and deliver and the team is extremely motivated and extremely hungry to maintain the run of success we have had in the last few years. There is a great determination in the team to defend both of these titles we have had in the last few years."

Horner did however admit that the season ahead would be far from easy from Vettel as he looks to emulate countryman Michael Schumacher in winning the title for a fourth season in a row.

"Things are different these days - we have the same tyres, testing is limited - it is a much more constricted set of regulations than in Michael's time," he said. "It is going to be massively tough - it is a healthy period for Formula 1 and the quality of drivers is simply superb at the moment - and for Sebastian to have achieved three world titles already is something remarkable. But he is going to keep pushing very hard to maintain that title for another year."