It was time to go, says Webber Mark Webber at last year's South Korean Grand Prix. Mark Webber's decision to retire from formula 1 was made more than six months ago. And it had nothing to do with his latest and most controversial run-in with Sebastian Vettel at the Malaysian Grand Prix in March. Webber confirmed that he had made up his mind to leave the front-running Red Bull team at the end of this season and switch to sports car racing in 2014, just before last Christmas. "It's been on the cards for a long while, towards the back end of 2012 for myself," he said after Thursday's announcement that he would lead Porsche's return to international sports car racing from next year. Webber revealed that his deal with Porsche, which the German car maker said extended for several seasons, had been finalised before the Australian Grand Prix in Melbourne's Albert Park in mid-March. "The decision was made before Christmas," he said. That's why I had all you guys [selected members of the Australian and British media] out for dinner in Melbourne. I knew that was my last race in Australia. The decision had been made well and truly by then. I didn't have to agonise over the decision. I was strong in myself. No one else can make the decision for you. You can't sit down and justify it with somebody else. It was up to me alone. He was adamant that Vettel's ignoring team orders and taking over the Malaysian GP - which Webber was set to win - had no bearing on his move to Porsche. The company is returning to sports car racing after a long absence, aiming for success in the Le Mans 24 Hours classic. But, no, Malaysia was not the reason, he said. There are lots of people I had Christmas with who can tell a different story. Webber explained that he had decided to end his F1 career after 12 seasons, during which he has so far won nine races, because he realised that his time as a leading driver was coming to an end. "You cannot do this for ever, and it's now my time to go to the new challenge," he said. It's a decision which in the end becomes easy because your tummy keeps getting stronger in terms of that feeling. You can't be at this level with the motivation not at tip-top. You need to be performing well, and I still believe I can do that before the year's out, but it's getting on the limit now, and you have to be fair and open with yourself. I'm completely comfortable with that. I'm looking forward to the new chapter, new challenge. I'm also looking forward to doing the remainder of the season and trying to get that Australian national anthem played again. Webber, who will be 37 on August 27 and is the oldest driver in F1, also admitted that he was mindful of leaving F1 when he was still competitive rather than when his form had declined to the point where he was no longer a regular force. "I have been mindful of that," he said. I've been very, very lucky to have very, very good sportsmen and women in my corner in Australia and over here as well in terms of openly saying they f---ed it up. They went too long and the memories aren't great. Or they got it right, or they were a bit early, whatever. It is one of the hardest decisions you have in your career, but you get to a point where your tummy just gets so strong and overwhelming that it's right. Webber had opportunities to continue in F1, but he was not confident he could secure a minimum two-year deal to justify staying, or that he could still be among the pacesetters in 2015. "I could have, but for me to do a multi-year deal somewhere, I think I'm going to struggle to get my head around where I'm going to be in 2015 ... to go and do another two seasons of that," he said. So it needed to be two seasons. For me to go and do a one-off somewhere is not going to work. Before you know it you're at Monaco [early in the season] and the same retirement questions are being asked of you. I know the drill. I've been here for a while, and you have to be open and honest with yourself. Yes, you can cling onto options and have future chances to operate here, but the category's going to be very different next year, so I may as well do the different category that is my future.