LEADING experts say the SNP could inflict serious damage on the Tory government if the party waged a lengthy campaign of sabotage and guerrilla warfare at Westminster.

Top academics and historians have said the constitutional logjam over Brexit in the Commons resembles that of the late 1970s, when a minority Labour government depended on SNP MPs and other smaller parties to keep it in power.

The SNP has now vowed to "frustrate what the government are doing as much as we possibly can".

The Sunday Herald spoke to experts, who set out contrasting tactics the SNP could employ in the weeks and months ahead at Westminster.

Leading Scottish historian Michael Fry said SNP MPs should adopt aggressive and robust tactics to frustrate the Brexit bill and Tory government at every turn.

However, James Mitchell, professor of public policy at the University of Edinburgh, said that "periodic unusual stunts" would be more effective.

The parliamentary filibuster

Fry said SNP MPs should "filibuster" in the Commons chamber - an approach that allows politicians to talk at length during a debate on a bill until parliamentary time runs out.

He said that could delay the legislation and cause havoc for Theresa May's Brexit timetable.

Fry said the tactics cause "huge damage" to the Tory government.

"The basic tactic is the filibuster," Fry said. "Those organising the filibuster do need to keep on the right side of the speaker, so will have to be masters of procedure."

Overnight and late sittings in the Commons

Fry said the SNP should force overnight sittings to prevent Commons business starting the next day and delay Brexit legislation.

He said: "The tactics could be especially effective in conditions like the present, when the government is always unsure of a majority.

"They need also to be well organised, ready with a roster of contributions and prepared to stay up all night.

"If the parliamentary SNP is disciplined enough to do all this they could well do huge damage to the Tory government which is on an extremely tight Brexit timetable.

"For Theresa May it's another adverse factor in a complex situation she barely controls, and maybe it will be the straw that breaks the camel's back."

Obstructionist tactics of Parnell's Irish Nationalists

Party insiders have said the obstructionist tactics of the Irish Nationalists under Charles Stewart Parnell in the 19th century should become the “template” for the party at Westminster in its bid to frustrate UK Government business.

The tactics could involve creating a mass of amendments to bills and calling for unnecessary votes; tactics designed to clog up the Westminster system.

In Commons exchanges, SNP MP Stewart McDonald invoked the spirit of Parnell, who led the campaign for Irish Home Rule in the 1880s.

McDonald told MPs: “Parnell, of course, along with his colleagues at the time, successfully used the standing orders of the House to frustrate Government business – which is an entirely legitimate tactic – to force them to take Irish issues seriously, including in a 45-hour sitting on the 1877 South Africa Bill."

Former justice secretary Kenny MacAskill, the former Scottish Government Justice Secretary, says that Parnell’s Irish Parliamentary Party should now be the “model” for the SNP at Westminster.

Set piece parliamentary stunts

Mitchell said that the most effective tactic would be to use shock and unexpected stunts, similar to Alex Salmond's famous interruption of the budget of the then chancellor, Nigel Lawson, in 1988.

He said Salmond's tactic had worked because it was tied to an issue - the poll tax and a cut in taxation for top earners that Lawson had announced.

He said: "Salmond 's protest worked because it was linked to what happened in the budget and the redistribution of wealth to the rich.

"It wouldn't work each week to try to disrupt parliament.

"There's always the danger that it goes too far and ends up losing public sympathy.

"If it's tied to issues like welfare and Brexit, that could work."

Meanwhile, Mitchell said the situation facing Theresa May's government, with backbench Tory rebellions and being dependent on the DUP for its survival, was reminiscent of that experienced by the minority Labour government led by James Callaghan in the late 1970s.

The Tory opposition of the day staged regular parliamentary ambushes, forcing unexpected votes in the Commons when Labour was not sure all its MPs could attend.

He said: "It reminds me of what happened in the parliament in the late 1970s with the Labour government."