Kezia Dugdale is facing the biggest battle of her leadership - against Jeremy Corbyn and the UK’s largest union.

Both are challenging plans to allow the Scottish Labour leader to appoint a member to the party’s powerful ruling body, the National Executive Committee (NEC).

The move is part of a package of reforms seen by Ms Dugdale as crucial to her plan to make Scottish Labour more autonomous.

Read more: Kezia Dugdale accuses Jeremy Corbyn of trying to undermine her

She hailed it as a personal victory when the changes were agreed by the NEC last week.

But the measures still have to be ratified by Labour’s annual conference in Liverpool this week.

Now they have become the subject of a turf war between supporters and opponents of Mr Corbyn.

Labour sources accused Mr Corbyn of trying to delay the creation of new Scottish and Welsh NEC seats during an meeting of the group on Sunday night.

Yesterday Len McCluskey, the leader of the Unite union, turned up the heat saying that a new Scottish representative should be voted for by members, not appointed by the Scottish leader.

Ms Dugdale's supporters insist she has enough support to win the vote later this week.

But a Labour source said: “One of Jeremy’s first actions after promising unity was to try to divide the party on an issue that the NEC has already decided.

"He was defeated in his attempt and it is important that the whole party gets behind these important reforms to give the Scottish and Welsh parties greater autonomy”.

On Unite’s intervention, he added: “There is a lot of support for these measures within the trades union movement”.

Read more: Kezia Dugdale accuses Jeremy Corbyn of trying to undermine her

Mr McCluskey said that his union, Labour''s largest donor, believed that “there should be a seat for Scotland and Wales on the NEC but we believe that it should be an elected seat."

He said there was a "confusion" over what ‘appointed by the leader’ meant.

The union is expected to decide tomorrow (Monday) whether or not it will back the rule changes.

At the moment the reforms, which also include support for working class candidates, are expected to be voted on as a whole.

Mr McCluskey said: “It presents a difficult for us because we would have like to have spoken on the Scottish and the Welsh seats separately and explained the fact that we would like seats on the NEC but we want them done democratically.”

He added: “My belief is that this package of proposals is likely to go through on Tuesday. What Unite’s position will be is something you’ll have to wait and see.”

Labour sources said that as Labour leader Mr Corbyn gets to appoint three people to the NEC.

A number of Mr Corbyn’s allies were elected to the NEC earlier this year.

But the veteran socialist still faces a struggle to command a majority of the body.

Read more: Kezia Dugdale accuses Jeremy Corbyn of trying to undermine her

Former Scottish Labour MP Ian Davidson backed the call for members to appoint the Scottish representative.

He said: “Now that Jeremy’s re-election has given Labour across Britain a huge boost of ideas and supporters we in Scotland have to see if we can capitalise on this Corbyn surge.”

Before that Ms Dugdale should take the seat herself or give it to her deputy Alex Rowley, or former Scottish Labour Party, Johann Lamont, who famously complained that the Scottish party was being treated like a “branch office” of the Westminster party.

Shadow chancellor John McDonnell said that the discussion on Sunday night centred on whether a delay was necessary to decide if the Scottish seat should be chosen by the party leader or elected by members.

If passed, the reforms would give Ms Dugdale more control than any other Scottish Labour leader in history.

Under the proposed changes the Scottish party would be in charge of Westminster candidate selection in Scotland and policy for the first time.

The reforms are the result of months of work by Ms Dugdale.

But even before the latest challenge she faced accusations from party insiders that the changes were "cosmetic".