Fernando Alonso has revealed the "very poor" on-track action in Formula 1 was the main reason behind his decision to quit the championship for 2019 as he pursues "bigger challenges" in other series.

Two-time F1 world champion Alonso announced on Tuesday he would not be racing in F1 for 2019, but left the door open to potentially return with McLaren in the future should the team become more competitive.

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Alonso is thought to be chasing a move into IndyCar for next year as part of his bid to complete the 'triple crown of motorsport' by winning the Indianapolis 500, having debuted at the event in 2017 and won the 24 Hours of Le Mans on debut earlier this year with Toyota.

Speaking at Silverstone ahead of this weekend's FIA World Endurance Championship round, Alonso explained how the on-track action in F1 was no longer meeting his hopes for racing in the series, prompting him to quit.

"I have other, bigger challenges than those Formula 1 can offer right now," Alonso said.

"It’s a sport or a series that definitely has some very positive things I’ve been enjoying for 17 years or 18 years of my life. I think I achieved much more than what I dreamed when I started in 2001, and right now, the action on-track is not the one that I dreamed of when I joined Formula 1 when I was in different series, or the action on-track that I experienced in other years.

"Most of my announcement went to sadness or a sad moment, or frustration over the last couple of years for the lack of results. I’ve been racing for 18 years in Formula 1, I’ve won two [titles] - so arguably, 16 years of my life, I was frustrated. It was not the case, and it’s not the case now.

"I stopped because the action on-track in my opinion I feel is very poor. In fact, what we talk about more in Formula 1 is off-track. We talk about polemics, we talk about radio messages, we talk about all these things.

"When we talk so many times about these things, it’s a bad sign. It’s because the on-track action was very poor that weekend. That’s what I feel in Formula 1 now, and I think there are other series that maybe offer better action, more joy, and I think more happiness."

Alonso conceded that while McLaren's lack of performance on-track had played a part in his decision, he said he felt F1 was still more enjoyable in previous years when he was also unable to fight at the front of the field consistently, before repeating comments made throughout 2018 criticising the sport for being too predictable.

"When I was in 2003, 2004, 2008, 2009, 2011, I was not winning any single race in those years. It was difficult to predict what could happen now in Spa or Monza," Alonso said.

"Now we can write down what is going to happen at Spa or Monza. We can put the first 15 positions with maybe one or two mistakes. How predictable everything became is tough. We came to Barcelona, we test the first day in winter testing, and you know what you will do until November in Abu Dhabi.

"It’s tough. For me, it’s not too much of a problem because after 18 years, I achieved more than what I dreamed of. But for young drivers or different drivers, it’s tough, because they just hope that next year, the team does an unbelievable step, or they receive a call from one or two teams. So it became difficult for ambitious drivers.

"A driver with some kind of ambition, it will be tough for the future if things do not change."