BNP leader Nick Griffin will be denied entry to a garden party at Buckingham Palace "due to the fact he has overtly used his personal invitation for party political purpose through the media".
In a statement Buckingham Palace said: "This in turn has increased the security threat and the potential discomfort to the many other guests also attending."
Mr Griffin's personal invitation to the garden party on Thursday was issued to him as an elected member of the European Parliament. The palace statement said: "The decision to deny entry is not intended to show any disrespect to the democratic process by which the invitation was issued. However, we would apply the same rules to anyone who tried to blatantly politicise their attendance in this way."
The BNP leader was set to attend last year as a guest of a BNP London assembly member but pulled out after an outcry. In an e-mail to supporters sent on Wednesday, Mr Griffin said he would be representing "a million British patriots who vote for this party".
"This event shows just how far this party has come in the last few years but I won't be at the Palace for myself or my family. No! I will be there to represent the patriots who made this possible; I'll be there for you."
He asked them to suggest what he should say to the Queen if they spoke at the event.
He was set to attend the party with wife Jackie and fellow BNP MEP Andrew Brons. It is not clear whether Mr Brons will attend. The BNP leader was expected to be among the 8,000 guests at the event which will be hosted by the Queen and also attended by the Duke of Edinburgh.
Other members of the Royal Family who will be mingling on the Palace's lawns include the Duke of York, The Duke and Duchess of Gloucester and the Duke of Kent.