In a heated letter to Richard Leonard, she says that she was “shocked” to discover he had intervened in a statement written by Catherine Stihler and David Martin for the party’s guide to its annual conference, due to be held this weekend.
She claims that on the instruction of Mr Leonard, and without the MEPs’ consent, a description of Brexit being “a tragedy for our country” and their support for a second referendum had been deleted.
However, a Scottish Labour source said there had been a “genuine misunderstanding” and Mr Leonard had now written to both MEPs to apologise.
In her letter Ms Dugdale also expressed her disappointment in Mr Leonard’s own position on Brexit. He said last week that he would rather have a Labour deal to leave the EU than a second vote.
And she said it was an insult the combined 55 years of service from Ms Stihler and Mr Martin would not be recognised at the conference in Dundee.
It is believed Ms Dugdale shared her letter to Mr Leonard with her party’s MSPs in Holyrood and the seven MPs in Westminster.
A source close to Ms Dugdale said: “Kezia felt it was important that all elected members should know that their words can be changed and printed without their permission.”
In her letter to Mr Leonard, she writes: “I wish to formally complain about the way in which Catherine Stihler and David Martin have been treated ahead of conference. From what I can see from the agenda, there is no formal opportunity for them to speak or indeed a formal item to thank them for their combined 55 years of electoral service to the party.
“If that wasn’t bad enough, I was shocked to discover from Catherine that her statement in the conference guide had been amended without her consent. When that was challenged, she was told it was on your direction and that you had the final say on what was printed.”
The letter repeats the deleted section, which states: “Brexit is a tragedy for our country and for the workers and communities that Labour represents. That’s why David and Catherine fully support a People’s Vote with the option to remain in the EU.”
This section was replaced with: “The complete mess the Tories have made of Brexit means they are putting Scottish people’s jobs and our industries at risk. Labour will always put them first.”
Ms Dugdale said: “I find that wholly inappropriate and I can’t possibly understand why you would seek to censor her final words to party members, especially since what she states is party policy and has been since last September.
“Can I encourage you to please take urgent steps to rectify what I consider to be a considerable insult and ensure that we thank these two outstanding MEPs properly at conference.”
It is understood other Scottish Labour MSPs have backed Ms Dugdale’s letter. A spokesperson for Ms Stihler said: “Catherine is a long-standing supporter of a People’s Vote and will be making her views clear as a Labour Party member at this weekend’s conference.”
A Scottish Labour source said: “This was a genuine misunderstanding and Richard has written to Catherine and David to apologise.”