As the fall-out continues from last week's Conservative victory - which saw Labour reduced to just one MP north of the Border - activists are now demanding the party changes its position on the constitution.
It continues: "Should all attempts to secure a second referendum by legal and constitutional means be obstructed by the UK government, we support an escalating strategy of non-cooperation and civil disobedience against that government, pursued at all possible levels: from the Scottish Government legislating on, or refusing to cooperate with, matters ‘reserved’ to Westminster, to grassroots non-violent direct action within and outwith Scotland."
The letter concludes: "We refuse to abandon the people of Wales, Northern Ireland and England to Tory rule, even if Scotland chooses to pursue its own road to socialism.
"We believe that real self-determination can only be achieved through cooperation between movements and across borders. We call on the left across the UK to back our demand for a second referendum on independence and to fight with us to secure a radical democracy for all the peoples of these islands."
Ewan Gibbs, a Glasgow party member who helped draft the letter, told The Scotsman: "The aim is to start a conversation at this point.
"I think the letter probably reflects the view of a substantial portion of the younger members that joined in the last few years and who were very active in the election.
"I would add that clearly some older trade unionists and more senior representatives agree that Labour needs to have a credible democratic position on a second referendum."
It follows the intervention yesterday of several senior Labour figures who declared they were in favour of allowing Holyrood to stage a referendum on Scottish independence.
The party's health spokeswoman, Monica Lennon, said the Scottish Parliament must be allowed to decide on the issue, while defeated Rutherglen MP Ged Killen said the SNP now had a mandate for a vote on leaving the UK after winning 47 of 59 seats in Scotland.
Scottish Labour leader Richard Leonard has also indicated the party would be looking to shift its approach ahead of the 2021 Holyrood elections as he announced a review of the party's disastrous election campaign.