Labour claims tonight’s event at Glasgow’s 02 ABC amounts to “treating”, a corrupt practice forbidden by the Representation of the People Act.

The 1983 law says a person is guilty of treating if, before, during or after an election, they provide “any meat, drink, entertainment or provision … for the purpose of corruptly influencing … a person to vote or refrain from voting”. An elector who “corruptly accepts” the offer is also guilty and faces a fine of up to £5000 or a year in jail.

Free tickets for “Midge Ure in Concert Live with the SNP” were only available through an SNP website, which asked for name, address and phone details, and if people wanted to volunteer for the party.

If the concert was found to be treating, the venue could be refused a liquor licence.

The SNP last night denied it was breaking the law, and insisted the concert would go ahead. The party announced the event earlier this month, when Mr Ure, co-organiser of Live Aid, endorsed Alex Salmond for re-election as First Minister.

Mr Ure said the gig would help get people involved in the Scottish election.

His support act Lou Hickey added: “It’s never been more important for everyone in Scotland to demonstrate their own independence by voting in this crucial election.

“We both hope that the night will play a part in encouraging a healthy turnout on May 5.”

QC Gordon Jackson, a former Labour MSP, said such free entertainment was virtually unheard of during elections because of the law on treating.

He said: “It’s very unusual, and raises questions about the ethics and legality of what the SNP are doing. Imagine offering people free beer to vote. Offering free entertainment is not in principle any different. It sounds very iffy.”

Guidance issued by the Electoral Commission and Association of Chief Police Officers in Scotland identifies treating as one of the “key offences” under the 1983 Act.

It requires “corrupt intent”, and does apply to incidental hospitality.

An 1892 case established that if people “are gathered together merely to gratify their appetites and so to influence their votes, then it is treating”.

Willie Bain, Labour MP for Glasgow North East, said: “The purpose of holding this concert is clearly to influence how people vote -- you have to go on to a website called votesnp.com, and that is treating.

“The really embarrassing part is that those innocent people who attend run the risk of getting into trouble and the venue’s licence could be at risk too. It is deeply, deeply irresponsible of the SNP.”

An SNP spokesman said: “Labour are the ultimate party poopers -- their negative campaign has crashed out of the charts by trying to stop Midge Ure singing.

“Gordon Jackson is old enough to remember the Red Wedge Labour election gigs -- their problem now is that young people wouldn’t want to go to an Iain Gray party.”

The venue did not respond to calls or emails.