Hon. Members are right to probe the issues through their amendments, because the law is being changed, but I hope to be able to give them some reassurance.

I must resist amendments Nos. 251 and 305 because the national security designation category for the new offence of trespass on a designated site would be removed from the Bill, and the Government believe that it is essential to retain that category to enable us to designate sites as such. However, I understand hon. Members' misgivings about the width of the definition of national security.

It is general Government policy, not just of the current Government, not to define national security, as that could hinder our flexibility in dealing with new and emerging threats. A single rigid definition could prevent us from dealing with issues that arise. It has been recognised by the courts, both here and in Europe, that the definition of national security is primarily a matter for the Executive, in this case the Secretary of State through the order-making power of designation. In addition, the House of Lords ruled in the case of Rehman in 2001 that national security embraces a precautionary approach, that is, potential threats as well as actual threats, which gives the Executive fairly wide leeway for making definitions.

We envisage being able to designate land belonging to the Crown, land belonging to the Queen or the Prince of Wales in their private capacity, and any land that falls under the national security designation. The provisions therefore cover places like Windsor castle and those parts of Buckingham palace not open to the public—clearly, some parts of the palace are open to the public, and as long as the public keep to those areas they will not be committing an offence. I hate to legislate in a reactive fashion, but hon. Members will be aware of the recommendations in the Armstrong report that followed Aaron Barschak's intrusion at Windsor castle and the security commission inquiry's report into Ryan Parry's intrusion into Buckingham palace. The Clause is a result of those various security authorities' recommendations to us. The legislation is designed to address a particular set of circumstances,   specifically mischief, although we are not in the business of using that as a catch-all to enable a wide designation of sites across the country.

I am concerned about the amendments relating to public access, because they would prevent sites where there is such access from being protected. The provision for a defence that people did not know the site was designated would be sufficient for them not to be prosecuted. Not only is there a defence, but the prosecution would have to have a realistic chance of success, which will militate against frivolous prosecutions of people who have unwittingly trespassed on a designated site. We have tried to draw the legislation to ensure that it is not a massive extension of restrictions on people's ability to enter various sites.

We do need a deterrent and an incentive for people to leave a designated site. The hon. Member for Beaconsfield made a genuine point about the need to put up signage so that members of the public actually know that the site is designated and do not accidentally wander in and find themselves committing a criminal offence. There is provision in the legislation to allow the Secretary of State to put up signage. The various sites will be designated through order, which will need to be laid before Parliament.

This is a fairly modest set of proposals, designed to attack the particular mischief of people getting into Buckingham palace and Windsor castle. It may well be that, in future, it will be considered appropriate to designate the Palace of Westminster on grounds of national security. Clearly, that will be a matter for consideration of the Secretary of State when drawing up the designated orders. As I think that the amendments would undermine the purpose of the legislation. I ask the hon. Gentleman to withdraw them.