The Duke of Cambridge has begun an official visit to Malta as a last-minute replacement for his pregnant wife as the country celebrates the 50th anniversary of its independence.
The trip would have been the duchess's first official solo overseas visit but she withdrew on doctors' advice after suffering severe morning sickness.
Prince William told Malta's President Marie-Louise Coleiro Preca that his wife was feeling "so-so".
Prince George has remained at home.
Malta gained its independence from the UK on 21 September 1964.
Peter Hunt, BBC Royal correspondent
This weekend was meant to have marked the next stage in the Duchess of Cambridge's on-the-job royal training - a solo visit abroad.
But the subtle shift in what she does will have to wait as a pregnant Kate continues to suffer, for a second time, from very acute morning sickness.
Prince William - who is becoming an old hand as he tiptoes towards his destiny - is stepping into the breach to the delight of the president of Malta.
Marie Louise Coleiro Preca told the BBC William's presence would give the 50th anniversary celebrations the "significance" that was needed. She also expressed the hope that he would return, next time, with Kate and George.
With a future king at the heart of the events, this will be an occasion when the talk will be of the benefits of independence - Maltese, not Scottish.
The couple made the decision on Wednesday that the duchess - who has a severe form of morning sickness - would not travel to Malta.
Catherine was forced to cancel a number of engagements last week after her second pregnancy was announced last Monday.
The itinerary for the trip was not changed after it was announced that Prince William would be travelling to the country instead of his wife.
Prince William told Mrs Coleiro Preca that the duchess was "so sad not to be here", adding "unfortunately, you have me".
During a tree-planting ceremony at the president's official residence, the prince commented on a marble plaque bearing his name that commemorated the event, joking: "Did that have to be re-written?".
He also told the president: "I was asking my grandmother all about her time in Malta just a week ago. I'm very interested in the history."
The Queen lived in Malta at various times between 1949 and 1951 when the Duke of Edinburgh was on naval service.
On the first day of his trip, the prince watched the re-enactment of a historical event in the capital Valletta with Mrs Coleiro Preca and Prime Minister Joseph Muscat.
The In Guardia Parade sees the fort's garrison dress in medieval costumes to act out the inspection of a fort and garrison by the grand bailiff of the Order of the Knights of St John.
Prince William was shown documents in Malta's national library, including letters from his royal ancestors Henry VIII and George II.
He also saw the original citation awarding the George Cross to Malta for its bravery in facing a sustained bombardment by Germany and Italy during World War Two, when it was a vital British base in the Mediterranean.
Crowds had gathered outside the library to welcome the prince to Malta.