The National Trust is to issue a warning to all the political parties over the use of its sites in campaigns after the extremist British National Party filmed on one of the charity's properties without its permission.
The trust was forced to take action when the BNP used the White Cliffs of Dover as a backdrop to a party political broadcast by the party's leader Nick Griffin.
During the broadcast, which was screened on 28 May, Griffin was shown discussing the issue of asylum seekers while standing on the cliffs overlooking the port of Dover.
A spokesman for the National Trust said that the charity had not been approached by the BNP for permission, and it would not have been granted in any case.
He said: "Our charitable status prohibits the use of National Trust land for political purposes. We would have had to decline the request, but we were not given that opportunity."
The spokesman said that the trust had not yet been in contact with the BNP, but it would be contacting all political parties to prevent it from happening again.
"The BNP was allocated one broadcast slot and there was very limited recourse for us because it had already gone out. As a charity, we have to be scrupulously above board about associations with any political party, and we need to make sure that all political parties are aware of that."
A spokeswoman for the Charity Commission said that this was an issue for the National Trust, which is entitled to enforce any rules it has about access to its property. However, if the charity had given permission, it could be in a position of breach of trust concerning engagement in political activities, which would be a matter for the regulator.
Last week, a spokesman for the BNP described the trust's reaction as "pathetic and silly".
The National Trust is responsible for more than 612,000 acres of countryside, 600 miles of coastline and over 200 historic buildings and gardens.