The classical music star, 35, has sung for royalty on numerous occasions, and will play at the Queen’s 90th birthday pageant at Windsor Castle, England next month (May16).

However a proposal to feature the monarch on a track from Katherine's new album Celebration was flatly refused by royal officials.

Record bosses wanted to use a short speech the Queen gave in the '50s at the start of a song about her titled The Mother's Heart. The idea was rejected however as Buckingham Palace has strict guidelines on the use of recordings of the Queen for commercial purposes.

"We don’t agree to the re-use of extracts from Her Majesty’s speeches for broadcast purposes, except for documentaries of national significance," a palace source tells Britain's Daily Mail newspaper.

A friend of the star insists she was not to blame for the misunderstanding, saying, "This wasn’t her idea. She didn’t even know about it. Someone at her record label thought it would be a nice idea to use the Queen’s speech, but, in the event, it wasn’t possible."

A separate source admits the episode was embarrassing, adding, "It’s rather humiliating,I think Katherine’s team were getting ideas above their station."

Despite the royal rejection, Katherine is set to lead the tributes to the British monarch in her 90th year. In addition to headlining the Windsor Castle celebration she will be performing her own tribute concert to the Queen at London's Royal Albert Hall in June (16).

The singer has also donated an advance payment for her Celebration album to the Queen’s Diamond Jubilee charitable trust.