SCOTTISH Labour’s longest-serving MP has launched a ferocious attack on his own party by saying that it is full of “thugs and incompetents”.

In a leaked WhatsApp message, Ian Murray made the criticism after his political ally Kezia Dugdale announced she was quitting Holyrood.

However, a party source described Murray's comments as a “disgrace” and urged him to stop “disparaging" party colleagues.

Scottish Labour insiders are bracing themselves for a disappointing European election campaign after polling showed the party had slumped to 14%.

Such an outcome would result in Labour losing one of the two seats in the Brussels parliament they won four years ago.

The party’s problems deepened last week over a row about the departure of Dugdale, who is leaving the Scottish Parliament for a university job.

Supporters of Dugdale said that her decision was partly informed by UK leader Jeremy Corbyn’s support for Brexit, a position echoed by Scottish leader Richard Leonard.

The spat played out last week on a WhatsApp group used by MPs, after one individual said it was a “pity” for the staff in Dugdale’s office who would lose their jobs working for her.

Murray, a centrist who represents Edinburgh South, staunchly defended Dugdale, who has also attracted sympathy after her party stopped supporting her financially when she was sued by a pro-independence blogger.

He wrote: “Her staff are fully in support of her decision as I understand it. The key issue here is that some, who now run the Scottish Labour Party, spent a lot of her time as leader trying to get rid of her and when they were successful they’ve spent the time after her leadership trying to force her out.

“Final straw was the leadership pulling her court case funding despite promises not to. Ultimately she won her case and the party lost £80,000 as they would’ve got their costs had they kept their promise.”

He added: “The party in Scotland is full of thugs and incompetents and this is just the latest in a sad tale that sees us on 14%.”

Murray’s comments came days after other WhatsApp messages, which were also leaked to this newspaper, showed Scottish Labour MPs openly complaining about the party’s Brexit strategy.

The MPs were angry that a draft European election leaflet made no reference to the party supporting a second referendum on Brexit.

Glasgow North East MP Paul Sweeney, who is Corbyn’s Shadow Minister for Scotland, wrote: “Let’s hope the NEC [Labour’s governing body] kill this bulls*** line.”

Ged Killen, who represents Rutherglen and Hamilton West, informed the same group: “I agree, the leaflet is a disaster and I can’t endorse it. The party is in crisis in Scotland and I think we need an urgent plan to address this.”

Martin Whitfield, the MP for East Lothian, and Murray also criticised the leaflet, after which a staffer asked if the politicians could take their conversation elsewhere:

“I think that talking about the party being in crisis in this forum is inappropriate. I appreciate that there are frustrations within the group, but this discussion should not be being conducted in full view of staff,” he wrote.

An SNP spokesperson said: "Labour members are clearly just as fed up as voters are with the party, which continues to side with the Tories on any number of issues.

"Is it any wonder that Labour are languishing in the polls? They lack any credibility, and are clearly more interested in saving face than doing the right thing.

"If Labour MPs can't trust the party leadership, why should the people of Scotland?"

A Scottish Labour spokesperson said: “We don’t comment on leaks.”

Murray said: "The fact that a colleague has chosen to leak this private conversation between Scottish Labour MPs is disappointing and demonstrates how low some will stoop by attacking fellow Labour politicians who call out the dreadful treatment of Kez.”