KEZIA Dugdale has declared war on Richard Leonard over Brexit with an attempt to inflict a damaging defeat on his leadership of Scottish Labour.

Last night, she was accused of seeking to overshadow her successor with the launch of a campaign in support of single market membership. Leonard responded that he was "quite relaxed" about it and that he supported debate and dissent.

The dramatic move sets the scene for a fierce clash over the EU at Scottish Labour's annual conference in Dundee this week.

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Party delegates will be asked to support the stance of her campaign – Scottish Labour for the Single Market.

Dugdale's challenge to Leonard came after Jeremy Corbyn backed the UK being in a permanent customs union with the EU after Brexit, a position which Leonard supports.

Corbyn argued that this would ensure free-flowing trade for business between member nations without making companies pay export taxes, or tariffs, at the border.

In his first significant Brexit intervention since Corbyn's announcement, Leonard told the Sunday Herald that a customs union would protect Scottish jobs and trade, hailing Corbyn's policy shift as "sensible" and "positive".

He said he was hopeful Corbyn's position would "prevail" at the conference, which opens on Friday.

However, Dugdale's campaign claims that Corbyn's policy shift did not go far enough.

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She supports single market membership, which allows the free movement of goods, services, money and people as if they were part of a single country.

She and her allies will back a move at the conference to seek to force Scottish Labour to adopt that position, a direct challenge to both Leonard and Corbyn.

The campaign is co-chaired by Dugdale, Edinburgh South MP Ian Murray and Catherine Stihler – a Scottish Labour MEP.

It claims that single market membership is the only way to tackle austerity, protect jobs, and defend workers' rights.

Dugdale said: "Jeremy Corbyn’s welcome decision to support a customs union puts clear blue water between us and the Tories, however – if we are to leave the EU – the least-worst option for limiting the damage caused by a Tory Brexit is to also remain in the single market. This is the only way to tackle austerity, protect jobs, and defend our hard-won rights for workers and consumers.

“Scottish Labour can lead the way and help persuade our party to ensure the UK permanently remains in the single market.”

However, Leonard' supporters said the campaign was a deliberate attempt to upstage him at his first conference.

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Pat Rafferty. the leader of Scotland's biggest trade union, Unite, accused Dugdale of seeking to undermine him.

He said: "It's a continuation of people trying to undermine the leaders of the Labour Party at Scottish and UK level.

"I'm pretty sure that when Kez was leader someone launching a campaign a week before Scottish Labour conference wouldn't have been something that was welcome."

A Scottish Labour source said Dugdale's reputation was in "shreds" after her controversial appearance on ITV's I'm A Celebrity, for which she received a £70,000 fee.

The source said: "She gave up her right to be leader of the Scottish Labour Party when she resigned, yet still she wants to be the centre of attention.

"She also took Labour to third place in Scotland. A period of silence on her part would be most welcome."

Dugdale's campaign launch is also a challenge to Corbyn, who will address the conference on the opening day on Friday. Leonard will make his keynote address on Saturday.

The conference will be asked to back a resolution in support of a soft Brexit. A number of constituency parties have passed motions supporting the UK "remaining permanently in the European single market and customs union”.

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The vote is scheduled to take place on Sunday, shortly before shadow chancellor John McDonnell delivers a speech.

Last night, Leonard said he was "relaxed" about Dugdale's campaign launch. Speaking to the Sunday Herald, he said he fully backed Corbyn's plans for a customs union.

"It's been a positive move by the Labour Party to make that statement," he said. "It's a positive view.

"He (Corbyn) was up in Scotland and we talked about the customs union issue. It strikes me as a sensible move for the Labour Party to support that. I hope that prevails at Scottish Labour conference".

Leonard said he also backed Corbyn's opposition to the UK remaining in the single market after Brexit.

"I wouldn't completely rule it out, but it's unlikely (I would support it)."

He said he would not seek to block a vote on the single market at the party conference, saying: "I'm quite relaxed about the launch of that campaign. I'm a strong supporter of debate and dissent."

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The TSSA transport union, which has campaigned for a soft Brexit, said it would support Leonard in any vote.

General Secretary, Manuel Cortes said: “By committing to a customs union with the EU Jeremy has cut through the constant Tory bicker and put concerns of ordinary people about their jobs and livelihoods back at the heart of Brexit."