Sebastien Ogier gave Ford an ultimatum to back M-Sport's World Rally Championship effort more, and its failure to do so prompted his decision to leave for Citroen.
Ogier and M-Sport won the 2017 WRC titles as effective privateers, and appeared set for more substantial Ford support this season.
But the fact that "Ford has not kept pace with our needs" left Ogier frustrated and caused his exit.
“At the end of last year, Ford seemed motivated to help us further and allow us to raise our level this season but unfortunately I was a little disappointed on that side," he told French newspaper L'Equipe.
"I gave Ford an ultimatum: either we had official support, or there was a good chance I would be interested to go somewhere else."
Ogier made clear that he had been reluctant to leave Malcolm Wilson's team.
“It was a difficult decision,” he told Motorsport.com. “It’s never easy to take a big decision like this one.
"I had two really good years with M-Sport and Ford and we achieved big things together.
"It’s no secret I felt good in this team and working with Malcolm and his guys was nice for me.
“It was not easy to leave them. On the other hand without more support from Ford it became more and more difficult to carry on.
"At the end it was my decision but it was a common decision with Malcolm and me that it was difficult to carry on.”
Ogier goes into the final three rounds of this year's WRC only third in the championship between the fully-factory Hyundai and Toyota programmes led by Thierry Neuville and Ott Tanak respectively.
He fears that his title defence will be over if he cannot beat both his rivals in the upcoming Rally Great Britain.
"If I want to keep my chance alive, I need to beat them this weekend," Ogier told Motorsport.com.
"In Spain and Australia, anything can happen. But this is a rally I like. It's a tricky rally with tricky conditions, but those conditions bring possibilities with them.
"I know I need to work on myself and do even better in the last three rallies."
The Citroen team Ogier is leaving for has not won a championship since the last of Sebastien Loeb's titles in 2012, finished last in the manufacturers' standings last season and currently occupies the same spot this year.
But Ogier said earlier this week he had been convinced by team principal Pierre Budar that Citroen was ready to raise its commitment and get its once-dominant WRC programme back on track.
Sébastien Ogier, Julien Ingrassia, M-Sport Ford WRT Ford Fiesta WRC
Photo by: M-Sport
Loeb joined Citroen for pre-Rally GB test
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Ogier gave Ford "ultimatum" before Citroen switch
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