Fernando Alonso has slammed the "poor" and "predictable" state of the current Formula 1 product as why he has decided to leave McLaren at the end of 2018.

This week, the Spaniard announced he would be calling time on his career after 17 years, though hasn't ruled out a future return, with fellow countryman Carlos Sainz confirmed as his replacement at the British team.

Speaking ahead of his participation at the Silverstone 6 Hours with Toyota in the WEC, which has been his extra-curricular activity this season away from the F1 grid, the 37-year-old admits a lack of action was influential in his decision.

"The action on track is not the one I dreamed of when I joined F1, or when I was in different series or the action on track that I experienced in other years," he explained.

"I stopped because the action on track in my opinion I feel is very poor. In fact, what we talk about more in F1, is off track.

"We talk about polemics. We talk about radio messages. We talk about all these things, and when we talk so many times about those things, it is a bad sign."

Also, with such a big gap currently between the top three teams and the rest, the double world champion sees little chance of that changing.

"When I was in 2003, 2004, 2008, 2009 and 2011, I was not winning [many] races in those years. But it was difficult to predict what could happen in Spa and Monza," Alonso claimed.

"Now, we can write down what is going to happen at Spa and Monza. We can put the first 15 positions with maybe one or two mistakes. So how predictable everything became is tough."

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The former Renault & Ferrari driver suggested he felt sorry for the good young drivers of today who were mired in mediocre teams unable to see a path to the front but believes now he personally can enjoy whatever he decides to do.

"I think the door open is more because I think I am driving at the best level of my career now," he said. "And why to close doors if anything could happen in the future?

"I am still young. I am not 45 years old. I feel strong and I am doing this year 27 races, so my thinking is to stop and that is why I stop. But who knows? That is the reason why.

The almost certain next step is expected to be a season of IndyCar possibly still with McLaren as he looks to win the Indianapolis 500 and become just the second driver in history to win motorsport's 'Triple Crown'.