Sebastian Vettel has this week found himself at the centre of ongoing speculation that he might soon launch an effort to return to Formula 1.

The quadruple world champion, newly retired in 2023, will be present at Suzuka next weekend for an undisclosed "project" - almost certainly related to environmentalism.

But it has ramped up reporters' questions about a potential F1 comeback.

"Will I come back? I can't say no because you never really know what can happen," the British broadcaster Sky now quotes him as saying.

His former boss at Red Bull, Dr Helmut Marko, added fuel to the fire this week by admitting he thinks 36-year-old Vettel's driving days are not yet over.

"I talk to him on the phone every now and then," Vettel told Der Westen on Wednesday. "We exchange ideas and are good friends.

"But at the moment there are no plans for me to come back."

Mathias Lauda, however - the son of the late F1 legend Niki Lauda - chatted with Vettel at the Nurburgring event last weekend about the German's potential comeback.

"Sebastian was there with his family," Mathias told Osterreich newspaper. "I wanted to know if he missed his Formula 1 days.

"He told me that he really misses the competition and certain moments, but that he doesn't miss the stress."

Vettel doesn't deny that.

He told Kolner Express newspaper: "I'm still coping well at the moment (with retirement).

"But nothing pushed me to the limit like Formula 1. That's what I miss the most. But at some point that adrenaline rush runs out, and then it's about moving on.

"I'm still searching and looking and this process is exciting in itself," said Vettel.