Allison, one of the most highly rated designers in the Formula 1 paddock, was viewed as a key asset for Lotus as it pushed to challenge Red Bull for title glory this year.
But just months after initial speculation that other outfits were trying to lure him away, high level sources at the outfit have revealed that Allison has handed in his notice at the Enstone-based team.
Although Lotus was unavailable for comment when contacted, it is understood that an official announcement confirming the news - and the appointment of Nick Chester as the team's replacement technical director - is imminent.
It is not known where Allison is heading, but the most obvious destinations would be McLaren, Mercedes or Ferrari.
However, McLaren has ruled out the possibility, and Mercedes already has a wealth of technical directors. That leaves Ferrari - which has been on a recruitment drive over recent seasons - as clear favourite.
Sources suggest that Allison has signed a contract forbidding him from revealing his new employer until it makes the announcement.
ALLISON FUTURE ALREADY A HOT TOPIC
Earlier this year, amid earlier rumours about Allison's future, team principal Eric Boullier warned against spending war among teams for top technical staff like Allison.
"I think it's insane to create a money war, when you start to pay your people crazy money," Boullier told AUTOSPORT.
"Formula 1 is very risky, it's very demanding, it's very exposed. There are big salaries in Formula 1 and most of the time, in every field actually, higher salaries than in any other industry. But what for?
"For me the company comes first, not fighting for a couple of hundred thousand pounds to keep an employee."
AUTOSPORT analysis January 2013: F1's tech silly season
It is too early to suggest what impact his departure will have on Lotus's season, but team owner Gerard Lopez told AUTOSPORT recently that only shock events - which Allison's departure certainly is - could scupper the team's chances of finishing in the championship top three.
"Honestly, bar any surprises, I always said that the aim is for the team to finish in the top three this year. People thought that I might be joking or exaggerating but that is where I think this team can go, and that is where we are right now," he said.
"We finished last year on the up, and we were pretty certain that we had a better car to start off with this year, with two drivers that have benefited from a full year of experience.
"The package was supposed to perform more at the beginning of the season, and it turned out it did.
"We are coming out with a fairly big package for Barcelona, which we hope is going to continue the trend and not unsettle the car."
Allison is a stalwart of the Enstone-based operation, having originally joined the outfit in the aero department when it was known as Benetton in 1991.
His sojourn was broken up by spells at Larrousse and Ferrari, but he was back at Benetton in between. His current stint began during the Renault years in 2005, and he became technical director four years later.
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