UKIP Scotland was last night at the centre of a cronyism row, after it emerged the son of the party’s biggest donor had been given a plum spot as a Holyrood candidate.

Calum Walker, 24, who was also made the party’s interim chairman recently, is to be the lead list candidate in the North East, where polls suggest UKIP may get an MSP in May.

Through two companies he controls, Walker’s father has donated £7000 to UKIP Scotland in the last two years, making him the party’s biggest donor since 2011.

Dundee-based David Walker’s firm Bruce Douglas Marketing Ltd gave £2000 in March 2014, and a second firm Ronatree Ltd gave £5000 in February 2015, according to official records.

Paul Henke, a UKIP supporter who has clashed with the leadership in the past, said of the Walker candidacy: “UKIP members will think that this is not right. The cronyism that is going on is appalling.”

Other UKIP candidates include Alan Melville, who was accused of spitting at a Yes campaigner during the referendum, and Robert Malyn, who apologised last year for cracking a joke about motor neurone disease.

Party MEP David Coburn is also embroiled in a double-jobbing row after he refused to give an assurance that he would give up his Brussels seat if he was elected to Holyrood.

Opinion polls show that UKIP, which launches its Holyrood campaign this week, could win seats in the Scottish Parliament. However, the preparations have been overshadowed by a row over the role of Coburn in the candidate selection process.

UKIP in Scotland has been in ‘special measures’ since 2014 due to infighting and Coburn told the Sunday Herald that the regional List rankings would be drawn up by party director Paul Oakden and his team.

This explanation was contradicted by Oaken who said Coburn would “devise” the Lists, which would then be considered by the party’s governing national executive committee.

UKIP published their candidate list on Friday and revealed prime slots for Coburn and his allies.

The MEP is the lead candidate in the Highlands and Islands, a region where UKIP is polling at nearly 10% – more than enough to install him as an MSP.

However, questions are likely to be asked about the vetting carried out by the anti-immigration party.

Melville, who is UKIP’s top candidate in the Lothians, was accused by Green candidate Sarah Beattie Smith of spitting at her in 2014.

The pair were general election candidates in Edinburgh last year. In a blog she said: "There was no LibDem on the panel tonight but we were joined by the UKIP candidate, Alan Melville. His presence surprised me, not because I hadn't expected the party to take part in hustings but because I hadn't previously put a face to a name.

"It was only upon walking into the room on the night that I realised that Alan Melville is the very same guy who, over 10 days in September, swore and spat at me on an almost daily basis as I ran the Green Yes Tardis on Leith Walk."

Melville denied the allegation at a hustings, but stood up and showed what he had done, which was a mock spitting gesture. "I never actually spat. I may have sworn,” he said at the time.

Malyn, who is standing in the West of Scotland, was slapped down by Coburn last year for making an offensive joke on social media.

Mocking the idea of former Prime Minister John Major leading the UK team for a re-negotiated settlement with the European Union, he wrote: "Its about as convincing as appointing Stephen Hawking as captain of the British Synchronised Swimming Team!”

He later said: “It was clearly wrong and I apologise for any offence given."

Sharon McGonigal, UKIP’s number one candidate in the West of Scotland, was also criticised last year after sharing a photograph of a boatload of naked women with the caption: “If Carlsberg did illegal immigrants”.

Speaking to the Sunday Herald, Coburn described as “ludicrous” suggestions that Walker’s top billing as a candidate amounted to cronyism.

“He is head of [UKIP youth wing] YI in Scotland. He is extremely good.”

However, he declined to say whether, in event of becoming an MSP, he would resign as an MEP: “I haven’t given it a great deal of thought. We have not discussed it yet.”

He also defended Melville as a “very good candidate” and said of McGonigal’s off colour retweet: “She wasn’t the first [to do such a thing] and she certainly won’t be the last.”

Iain McGill, a Conservative candidate in Edinburgh, said: “Now we know why UKIP were slow to announce their candidates. The people of Scotland will see through them and treat them with the contempt they deserve.”

Walker could not be reached,