The leadership of the Wimbledon- based Communication Workers Union (CWU) approved the stance last week after what one critic described as a "sham" consultation.
The position is expected to be ratified at the union's UK conference in Bournemouth next month.
Trade unions are beginning to decide their constitutional positions ahead of September's referendum.
However, fears have been voiced that Labour-affiliated unions are effectively bypassing their Scottish members in a bid to endorse a No vote.
The CWU represents over 200,000 workers across the UK and nearly 15,000 in Scotland, including staff in the Royal Mail, British Telecom and Virgin Media.
Its consultation began last year with seven meetings across the country, at which members heard speakers from the Yes Scotland and Better Together campaigns.
The forums were hosted by CWU deputy general secretary Andy Kerr, a lifelong Labour member who sits on the party's national executive.
No votes were taken at the meetings, which were intended to take the pulse of the union rank and file.
Around the same time, the CWU published an independence briefing that Yes campaigners believed was slanted.
The CWU followed the meetings with two telephone polls, which the union's spokesman said covered around 1600 people in Scotland - around 10% of the membership north of the Border.
He said just over 60% of those polled favoured a No vote.
A motion in the name of Kerr and Billy Hayes, the union's general secretary and also a long-standing Labour member, was tabled at the CWU national executive and approved.
It recommended CWU members vote No in the poll.
The motion, a copy of which has been obtained by this newspaper, stated that local branches were free to make their own views known, but noted that the union nationally had its own recommendation: "Conference therefore endorses the NEC decision to recommend to all members in Scotland that they vote 'no' in the forthcoming Scottish independence referendum and appropriate communications are circulated to our Scottish membership to this effect."
The union's national conference, rather than a special conference of Scottish branches, will make a final decision next month.
One source close to the union said: "It was a stitch-up. The consultation was a sham."
Since 2001, the CWU has donated several million pounds to Labour.
Colin Fox, co-convener of the Scottish Socialist Party who took part in the CWU debates, said: "There was always a strong suspicion that the consultation was not as honest and as sincere as it should have been."
A CWU spokesman said: "The NEC has recommended a No vote after a wide-ranging consultation."