London, Jan 26 (IANS) The British royal family has sent "warning" invitations for Prince William's wedding to European royals by fax, asking them to keep April 29 - the wedding date - free of other engagements.

Although traditional gold-embossed and handwritten invitations will be sent next month to over 1,800 guests by the Lord Chamberlain's Office - responsible for organising all court functions - protocol says that sovereigns are given earlier warning than foreign heads of state and non-VIP guests.

According to senior aides, Buckingham Palace has faxed all European royals, asking them to keep April 29 free, the Daily Mail reported.

Palace courtiers said this was the most "efficient" way of passing on information about such "large-scale" events.

Some of the guests have already confirmed they would be at the Westminster Abbey ceremony.

King Constantine, the 70-year-old former sovereign of Greece, his wife Queen Anne-Marie, 64, and their eldest son Crown Prince Pavlov, 43, are to attend.

Constantine, who reigned from 1964 to 1973 when Greece was declared a republic and monarchy was abolished, is a godfather of William and a second cousin and close friend of Prince Charles.

Serbia's Crown Prince Alexander, who is the queen's godson, and his wife Crown Princess Katherine, released a statement on their website, saying: "Their royal highnesses are delighted to attend the marriage and are very happy for the young couple."

The Romanian monarchy, led by 89-year-old former King Michael, a first cousin of Prince Philip, said they would also be among the guests.

Other royal houses, including Spain, Holland, Norway, Sweden, Denmark and Belgium, are also expected to be invited.