Denmark's Prince Christian accidentally punched his mother Princess Mary in the eye when he excitedly waved to royal fans from the palace balcony. The princess, her 18 month-old son and her husband Crown Prince Frederik joined Queen Margrethe to celebrate her 67th birthday at Copenhagen's Amalienborg Palace on Monday.

Just after noon, the royal family walked out onto the balcony to wave to thousands of well-wishers waiting to catch a glimpse of the popular monarch.

Christian enthusiastically copied his mother by widely waving his arms, inadvertently hitting the pregnant Australian-born princess in the face.

Queen Margrethe - who has ruled since 1972 - was also joined by her husband Prince Henrik, her son Crown Prince Joachim and her other grandchildren Nikolai and Felix.

It was recently reported that Joachim, 37, planned to announce his engagement to Marie Cavallier. Speculation grew after the prince took his girlfriend to Marselisborg Castle, near Aarhus, to meet his parents over the Easter holiday.

Joachim was previously married to Hong Kong-born Alexandra Manley but they divorced in 2005.

The prince shares custody of his two sons, Nikolai, six, and Felix, four, with their mother Alexandra.