Union claims it is the victim of a Labour 'witch hunt'
Unite boss Pat Rafferty said the party's right wing 'preach unity but practice division'.
A Scottish union chief has accused Labour party moderates of preaching unity but practising division.
Unite Scotland boss Pat Rafferty also claims the organisation is the victim of a "witch-hunt" over accusations of vote-rigging in the party's Scottish leadership election.
Mr Rafferty said Unite had not broken the rules in signing up members to vote in the contest, which is open to Scottish Labour members, registered supporters of the party and supporters of attached trade unions.
He spoke out after Edinburgh South Labour MP Ian Murray claimed Unite's method of signing up affiliate members is "against the rules of the Scottish Labour Party".
Mr Murray, who is backing Anas Sarwar MSP in the contest , added it seemed "clumsy and, at worst, being rigged for a particular process".
'Thousands upon thousands of Unite members have decided to exercise their entirely legitimate right to vote in the Scottish Labour leadership election. Unite has not broken a single rule.'Pat Rafferty, Unite Scotland
Unite is supporting his rival Richard Leonard MSP, a former trade union organiser, in the ballot which was sparked by Kezia Dugdale's resignation as leader in August.
Speaking ahead of Unite's Scottish Policy Conference in Aviemore, Mr Rafferty said: "Thousands upon thousands of Unite members have decided to exercise their entirely legitimate right to vote in the Scottish Labour leadership election.
"Unite has not broken a single rule.
"The Scottish Labour Party's general secretary has said so on numerous occasions."
He claimed the union is "being subjected to a sustained witch-hunt from the right wing of the Labour Party".
Mr Rafferty's conference speech is expected underline his rejection of the vote-rigging claims and accuse Mr Murray and his allies of trying to "preach unity but practice division".
He will also call on Unite members to back Mr Leonard and question Mr Sarwar's decision to send his children to a fee-paying school instead of state education.
Mr Rafferty's comments were criticised by Mr Murray and Mr Sarwar's campaign team.
Mr Murray said: "I don't respond to bullies, especially those too cowardly to even have a conversation to ascertain the facts."
A spokesman for the Mr Sarwar's campaign said: "This personal attack from Pat Rafferty reveals how little he cares about uniting our Labour family.
"His comments will dismay both party members and Unite members who reject division and want to bring our movement together so that we can kick the Tories and SNP out of office.
"We will continue to focus entirely on our positive campaign of progressive taxation, tackling child poverty and retaining membership of the EU single market.
"Pat Rafferty may not want to talk about policies in this contest, but the thousands of party members and trade unionists who back Anas's bold and radical agenda - and who want Labour back in power in Scotland - certainly do."