The senior source claimed it was now clear the First Minister had simply used the Brexit process, which have included meetings of the Joint Ministerial Committee between the UK Government and the devolved administrations, to further the independence cause.
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"She has never been interested in working with the UK Government in delivering the best Brexit deal for Scotland and the whole of the UK," he declared.
His comments came as it was made clear Theresa May's non-committal line - she has not yet said whether or not she will concede to Ms Sturgeon's call for the UK Government to facilitate a second poll - would hold for the time being.
The insider explained there had been many areas of common ground between London and Edinburgh in the pre-Brexit talks such as on the protection of workers' rights.
"This has always been about independence for the First Minister and today she was not speaking as First Minister but as the leader of the SNP; her constitutional tunnel vision has come to the fore. It's absolutely irresponsible."
He went on: "A second referendum would be damaging, divisive and a huge distraction from the priority of securing the best Brexit deal."
The insider added that Ms Sturgeon had "hid behind a cloak of vagueness" on what she was proposing an independent Scotland would do on the issue of membership of the EU and on its currency.
He pointed out how if Scotland voted Yes on the FM's timescale, then Scotland would be extracting itself from the UK as the UK was leaving the EU; that is, an independent Scotland would, for an unknown period, be outside both unions.