The FBU boss bad-mouthed his own Scottish secretary John Duffy - who is currently facing a leadership challenge - and tried to stop another colleague from speaking to the press.
Wrack was told he had an "obsession" with Duffy and was warned his behaviour towards another Scottish official was becoming "tantamount to bullying and harassment".
The damaging row can be traced to last year's decision by the firefighters union to strike against the UK Government's pension cuts.
However, the administration of pensions is devolved to Holyrood and the union's Scottish office won concessions from the SNP Government.
Union members in Scotland accepted the revised offer and agreed to shelve strike action.
It is understood Scotland's no-strike position angered Wrack, who wanted a UK-wide stance.
Duffy, a member of the SNP, has now been challenged for his job and the result of the contest is expected on Wednesday.
Emails obtained by this newspaper reveal the full extent of Wrack's sniping at the FBU leadership in Scotland - comprising Duffy and Roddy Robertson - and the pressure he has tried to exert.
In July, Wrack was incensed that the FBU in Scotland published a letter relating to the union's distinctive position on the pensions dispute.
In an email to Robertson, he wrote: "It is once again disappointing that you have acted on a pensions matter without even discussing it with me or with the national officer."
On Duffy saying that Scottish union members could get "dragged into" an overtime ban, Wrack said: "John seems to have forgotten that the Fire Brigades Union decided this campaign at a national conference and in two national ballots.
"Members in Scotland have not been 'dragged into' anything - they VOTED for industrial action."
He added: "I therefore must insist that no further comments are made to the press on the union's pension campaign without discussing it with us.
"If that means questions should be referred to Head Office - that would be preferable to the error which was made in these comments."
However, Robertson shot back: "The only one creating confusion in the membership in Scotland is you … I suggest that you take a step back as this is becoming tantamount to bullying and harassment."
Months earlier, after Scottish members agreed to shelve strike action, Wrack showed his frustration with what he called Duffy's "social partnership" approach.
In an email to Robertson, he wrote: "Social partnership weakness [sic] workers and their organisations and leads to trade union officials ending up putting forward the case for employers."
He added: "I think we need to change course as a matter of urgency."
He also blasted Duffy for accepting an OBE, saying: "I have to say I find it a huge embarrassment that an official of this union took such an honour from the British establishment at a time when we are facing the worst cuts and attacks in history."
Robertson responded: "Your obsession with John is unhealthy and clouds your judgement."
He added: "If you think that standing outside and doing our negotiations by press release is the best way forward then I will review my thinking."
Robertson concluded: "As for John getting an award, get over it."
Paul Holleran, Scottish organiser of the National Union of Journalists, said Wrack criticised the FBU officials in Scotland at an STUC fringe meeting earlier this year.
"Matt Wrack spoke about how both governments were as bad as each other. I told him the Scottish Government had tried to take a different position," said Holleran.
"At the end of the meeting, he said the problem was that his officials in Scotland were more interested in independence than standing up for their members. I told him I knew these guys well and that his [Wrack's] view was off the wall."
Duffy is being challenged for the Scottish secretary post by fellow FBU official Stephen Thomson.
The FBU has around 40,000 members across the UK.
Robertson told the Sunday Herald: "I have been chastised on more than one occasion by my general secretary about telling the members what the up-to-date position was with our negotiation in Scotland and have requested numerous times that our head office include information for the Scottish membership.
"It would appear, however, that some are happy to create a vacuum and allow those who are wishing to challenge John Duffy and myself all the space to attack us."
A spokesman for the FBU said: "Our democratically elected conference has set out our policy and approach to campaigning to defend the pensions of firefighters all across the UK. This is the policy which is implemented by the union's leadership.
"As in all organisations, there is discussion and debate in the FBU. The union and its members are united in fighting to defend our fire and rescue service against cuts, which threaten the safety of the public and of firefighters."