As reported in Friday's Scottish Daily Mail, the Paisley club has informed Ibrox chief executive Charles Green and manager Ally McCoist of their intention ahead of next week's formal vote of all 12 SPL clubs.
Six top-flight clubs have already gone public with plans to reject the bid, which requires an 8-4 majority, meaning Green faces the prospect of relaunching the club in the Scottish Football League.
Like others, Saints have cited 'sporting integrity' for the decision, despite admitting the financial implications could lead to job losses at the club.
St Mirren chiefs have also confirmed they are unlikely to sign any new players until the situation surrounding Scottish football becomes clearer.
Chairman Stewart Gilmour said in a statement on the club's website: 'It should be made clear to all St Mirren supporters that I met with Charles Green, Alistair McCoist and other representatives of Rangers Newco last week.
'I made it perfectly clear that our directors' view was that our club would not be voting for a Newco entry to the SPL, directly to the officials of Rangers Newco.
'This meeting was held in confidential circumstances to allow both parties to enable the other to see their respective positions.
'A very positive meeting was held. However, the position of St Mirren has not changed and our stance of sporting integrity was maintained against the very pressing commercial arguments.'
He added: 'Regrettably, the commercial, and hence financial, impact is still not clear to the club.
'We are very hopeful that this aspect will become clearer in the next few days.
'This will play a major part in the decision-making for the board of directors as to the way forward for the club in the short term.
'We are obviously hoping that our commercial and television partners stay with the league through this very difficult period.
'It has been impossible for the clubs to downsize as quickly as we would like due to the contractual obligations of the club.
'These are considerable and must be maintained to ensure the club does not slip into the fate of others.
'It is possible that major cutbacks may have to be made within the company.