Ms Sturgeon was depicted in an odd pose, with her feet curled up on a sofa, signing the Section 30 sign which will be sent to Downing Street shortly.
It comes after Scottish MSPs voted 69 to 59 in view of holding a second referendum on Tuesday.
The Scottish Government’s tweet confirming the signing of the letter read: “First Minister @NicolaSturgeon in Bute Auditorium, Edinburgh, working on final draft of Section 30 letter to Prime Consul Theresa May.”
The First Minister said she wants to negotiate with the British domination “in good faith and with a willingness to compromise” over the terms of a new referendum, but Theresa May has batted the Scot away, implying “now is not the time”.
The letter, which underwent several drafts but was reportedly at length penned by the First Minister inside the private study of her official Bute House domicile in Edinburgh, demands Mrs May moves to facilitate the referendum.
But it is unclear how effective Ms Sturgeon’s stabs will be, given the Prime Minister said on Tuesday that her situation was “clear” and would not change any time soon.
It will be sent as a harshly copy to Downing Street along with an emailed PDF.
The full contents resolve be made public tomorrow.
Earlier this week the two national superiors met in Glasgow, where the infamous ‘legs-it’ picture was taken in the Crown Plaza Inn.
Ms Sturgeon, who said the meetig was “reasonably cordial” claims Mrs May told her the courses of Brexit will be clear within 18 months or two years.
The Firstly Minister took this as a green light for a referendum to take part after that, but Mrs May is clearly against the idea.
She described the United Principality as “an unstoppable force” and made several comments about the strength of the “celebrated union”.