The Palace of Holyroodhouse, which would be covered by the law

But the move has sparked a fierce political war of words.

The SNP and the Association of Chief Police Officers said Scots law was already sufficient to deal with intruders on designated sites.

Ministers said it was necessary to coincide with legislation closing a loophole south of the border.

The move would ensure that national security was defended, they added.

Sewel motion

The Scottish Executive wants the Serious Organised Crime and Police Bill, now going through Westminster, to be subject to what is known as a Sewel motion.

This is a parliamentary device whereby Westminster can legislate on Scotland's behalf and the executive can then tag this onto its own legislation.

But opposition parties are already angry at what they see as too many Sewell motions - 55 to date.

This is unnecessary, unwanted and alien to Scotland Kenny MacAskill Scottish National Party

The offence of trespassing on a designated site is being introduced to address problems that police forces in England have had in tackling intruders on sites like Buckingham Palace and Westminster.

But Scottish National Party spokesman Kenny MacAskill said: "This is unnecessary, unwanted and alien to Scotland.

"Scots law is perfectly adequate in this area and it's contrary to where we're going in terms of right to roam."

The Scottish Conservatives stressed that the change did nothing to reassure them about the bill as a whole, which they want debated in full by MSPs.

'Terrorist threat'

Justice spokeswoman Annabel Goldie said: "I have to question whether these new measures are necessary.

"But it's only because such confusion was created under the land reform legislation that anomalies and problems like this are having to be addressed to ensure the Royal Family have the same security in Scotland as elsewhere in the United Kingdom."

However, Justice Minister Cathy Jamieson said: "In light of discussions with the Home Office it is now proposed to make amendments to the bill to extend to Scotland the offence of trespass on a designated site.

The Palace of Westminster would be covered by the English law

"What we are proposing is creating a new offence of entering or being on a designated site without lawful authority.

"This is a separate Scottish provision that has been drafted to take account of the clear distinction between Scots law and the concept of trespass in English law.

"We believe the public will recognise that special measures are necessary in relation to such sites, especially given the threat of terrorist activity."

The minister, who insisted the offence would not be called trespass in Scotland, said: "We are absolutely clear that nothing should be left to chance in terms of national security and the protection of the public."

Academics' view

The Association of Chief Police Officers said the new offence was not necessary.

James Chalmers, of Aberdeen University's law school, argued that the trespass measures would add little to existing Scottish police powers.

But Professor Kenneth Norrie, of Strathclyde University, said trespass was not alien to Scots law.

The right to roam was enshrined in land reform legislation passed in 2003 by the Scottish Parliament.

A proposed 66-page access code on the "right of responsible access" was drawn up by Scottish Natural Heritage

The code was generally welcomed by the Scottish Landowners' Federation (now called the Scottish Rural Property and Business Association) and the National Farmers Union in Scotland.