Updated 8.15am, 24 March: In a statement released this morning, GMG confirmed the departure of McCall after the story of her imminent move to easyJet broke last night.
A firm date for her departure is yet to be announced, as are plans to appoint a successor, but she will take up her new appointment later in the year.
McCall spent 24 years with GMG, four as chief executive. She began her career at The Guardian as a planner in the marketing department.
GMG said an announcement regarding her successor will be made in due course.
McCall, this morning, said she had been "incredibly privileged to be part of such a wonderful and unique organisation".
She said: "The chance to lead a FTSE company of easyJet's calibre is a once-in-a-lifetime opportunity. The business has a superb brand and a great future ahead of it, and I'm looking forward to driving its continued success.
"I will leave GMG knowing that our journalism has never been more influential or widely read, and with complete confidence that its financial position is secure."
The appointment of McCall as chief executive at Easyjet will come as a shock to the media industry.
Over recent times she has been forced to undertake major cost-cutting measures to stem the losses at Guardian News & Media, publisher of The Guardian and Observer newspapers, but at no point have rumours circulated that she would leave the company.
Most recently, GMG offloaded its regional media division, which included breaking its ties with its historic home of Manchester, as part of measures to safeguard the future of The Guardian newspaper.
In November GN&M announced it would make 100 redundancies and close a number of its supplements to alleviate losses running at about £100,000 a day.
However, during McCall's time at GN&M and GMG she has overseen unprecedented growth in the company's digital properties, with Guardian.co.uk now ranking among the top news websites in the world. The main paper shifted to the Berliner format and the company also relocated to new premises in Kings Place, London.
She oversaw the sale of a stake in Trader Media Group and the joint investment with Apax in B2B publisher Emap.
At easyJet, she takes over a company that has its own challenges, with the airline industry hit hard in recent times.
EasyJet chief executive Andy Harrison announced he was leaving the company in December last year, and earlier this month he was named as the new boss at Whitbread. The company has also seen changes of chairman and chief financial officer in the past year.
The airline confirmed McCall's appointment in a statement to the stock exchange this morning.
Michael Rake, chairman of easyJet, said the unanimous decision to appoint McCall followed a "thorough process involving a number of high calibre candidates".
He described the incoming chief executive as having "the strategic ability, operational capability and passion to drive easyJet through the next stage of its development".
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