Nicola Sturgeon has tweeted a photograph of herself working on an official letter asking Theresa May for powers to hold a fresh independence referendum for Scotland.

The photograph, in an unusually informal pose and setting, showing the first minister with her legs curled under her on a sofa at her official residence in Edinburgh, was sent out from Bute House on her official account – @ScotGovFM – in a deliberate echo of the images sent from Downing Street of the prime minister signing her article 50 letter this week.

The first minister’s tweet was captioned: “First Minister @NicolaSturgeon in Bute House, Edinburgh, working on final draft of Section 30 letter to Prime Minister Theresa May.”

Sturgeon’s letter, which will be officially released on Friday, asks May for section 30 powers under the Scotland Act temporarily empowering Holyrood to stage the referendum.

The first minister will underscore that request by reminding May that her quest for a referendum was supported by a 10-vote majority at Holyrood, on Tuesday, and based on a manifesto commitment in Scotland’s parliamentary elections in 2016.

May has repeatedly said the request would be turned down. On Wednesday evening, she told Andrew Neil, in a BBC 1 interview on Brexit: “Now is not the time to focus on a second independence referendum or to be looking at that second independence referendum, because [now] is the time when we need to pull together as a United Kingdom.”