The reboot of the hit motoring series Top Gear was more than a year in the making. But after just six episodes the wheels appear to have fallen off, with the show's lead host Chris Evans quitting.

"Stepping down from Top Gear," Evans said in a brief statement released on social media.

"Gave it my best shot but sometimes that's not enough. I feel like my standing aside is the single best thing I can now do to help the cause."

"I have never worked with a more committed and driven team than the team I have worked with over the last 12 months," Evans said via a BBC statement.

"I remain a huge fan of the show, always have been, always will be."

Meanwhile, in what is reportedly coincidental timing, London's Metropolitan Police confirmed they had received allegations of sexual assault against Evans dating back to the 1990s.

"An allegation of non-recent sexual assault was made to police on May 23," a police statement said. "The allegation was made by a woman against a man, and relates to incidents in Tower Hamlets in the 1990s.

"There have been no arrests and no person has been interviewed under caution."

The statement relates to allegations from a former colleague that Evans exposed himself to her and grabbed her breasts.

Evans has described the claims, first aired in May, as a "witch hunt". And in a vote of confidence in him from the BBC, he will continue to host his national breakfast show on Radio 2 during any investigation. 

Since the launch of the rebooted series, Top Gear ratings have been in freefall.

The show's 23rd season launched in May with 6.42 million viewers in the UK.

Every week since it has posted a dramatic loss, landing at 4.06 million, then 3.44 million, 3.22 million, 2.68 million and finally, for its sixth episode, just 1.9 million.

Mark Linsey, Director of BBC Studios, said Evans had done "everything he could to make the show a success".

"He firmly believes that the right people remain, on both the production team and presenting team to take the show forward and make it the hit we want it to be," he said.

"The new series has so far notched up over 30 million views in the UK alone and has already been sold to 130 territories worldwide."

Evans replaced the show's previous host Jeremy Clarkson, who was suspended in March, 2015, and later sacked, after a succession of scandals plagued the show.

Clarkson's co-hosts James May and Richard Hammond said they would not return without him, and the show's executive producer Andy Wilman also resigned.

Their departures triggered a major reboot of the series.

The show's three hosts were replaced with six: Evans, American actor Matt LeBlanc, former motorsport boss Eddie Jordan, motoring journalist Chris Harris, race car driver Sabine Schmitz and motoring journalist Rory Reid.

How Evans' departure affects the show's other hosts remains unclear.

A 24th season of the show is likely, as it is one of the BBC's most powerful revenue earners in terms of international program sales and product licensing.

But the broadcaster has not confirmed if any of the show's five remaining hosts will return for a new season.

The BBC had only commissioned six episodes – cut down from an earlier order of 10 – for the show's 23rd season.

All six of those episodes have now aired in the UK.

Speaking to Fairfax Media in Cannes last year where the BBC was launching the reboot, Evans said he had taken the gig with mixed emotions.

"When watched the last show, or what became the last show, which none of us knew, nor did they when they were making it, it was so sad," he said.

"It was like a patient that didn't know it was dying or hadn't been told."

While the BBC will insist it is smooth sailing, there is little doubt the broadcaster's richest programming asset is in crisis.

UK newspapers have, for some time, been reporting rumours of rifts on the set between Evans and LeBlanc as it has struggled in the ratings.

The audience flatline for the show's sixth episode represents the smallest audience for the show in a decade in the UK.

The Sun newspaper quoted unnamed sources on the "crisis-hit" program who said LeBlanc had delivered "a crushing ultimatum [that he would] have no part in any future series that features Chris."

While it might seem that Evans would retain the upper negotiating hand in that situation – he is also a BBC Radio 2 host – LeBlanc's appeal to an international television audience would strengthen his own position.

A perfect illustration of Top Gear's focus on its non-UK audience can be seen in the selection of celebrity guests for the show's 23rd season: a mix of Americans such as actors Jesse Eisenberg and Patrick Dempsey, comedian Kevin Hart and adventurer Bear Grylls, and UK personalities who have large US profiles, such as chef Gordon Ramsay, actor Damian Lewis and comedian Jennifer Saunders.

The Daily Express has also reported that the BBC was searching internally for a new producer to take over the series in an attempt to dilute Evans' creative control.

The news also comes as the US version of the series is being shelved by the cable History channel.

After six seasons, History axed the US version of Top Gear, hosted by Rutledge Wood, who then announced on social media the series was "done for the immediate future".

BBC Worldwide North America confirmed the US version of the series was ending, but said they were investigating their options for the future.

"We are fully committed to the hugely successful Top Gear brand and have begun exploring new opportunities for the series in the US," they said in a statement released to media.