The Emir of Qatar will begin a two-day state visit to the UK hosted by the King.
Charles will welcome Sheikh Tamim bin Hamad Al Thani and the first of his three wives, Sheikha Jawaher, during a ceremony of pomp and pageantry staged in Whitehall.
But missing will be Queen Camilla who, on Monday evening, pulled out of the official open-air greeting on Horse Guards Parade and a carriage ride back to Buckingham Palace, due to lingering effects of a recent viral chest infection.
The Prince and Princess of Wales will take part in the ceremony on Horse Guards Parade, one of the few official appearances by Kate during a year in which she has been successfully treated for cancer.
The princess has been gradually returning to royal duties after she announced in September she had completed her chemotherapy treatment and her focus was 鈥渄oing what I can to stay cancer free鈥.
She will travel with her husband in the carriage procession and the couple will join Charles and Camilla as they host a lunch for their guests Sheikh Tamim and Sheikha Jawaher.
However, Kate will not attend the state banquet hosted by the King, although William will be a guest.
She is hoping to be able to view a display of Qatari artefacts after lunch in the palace鈥檚 picture gallery with her husband, the Waleses and the Emir, and at the start of the state banquet will pose for a group photo but will take a short break before dinner while guests are met in a receiving line.
No major changes are expected to her diary for the rest of the week, but the Queen has been advised by her medical team to take each day as it comes.
Meanwhile, the occasion marks the start of a busier week for Kate, who will be hosting her annual Christmas carol service at Westminster Abbey on Friday, when she is expected to be joined by her children and husband for the event.
Sheikh Tamim, 44, was educated in Britain, attending public schools Sherborne and Harrow, before graduating from Sandhurst military academy.
On Wednesday, the final day of the two-day visit, the Emir will visit Sandhurst before travelling to the Palace to bid farewell formally to the King and Queen, with Camilla expected to be there.
Sheikh Tamim will also travel to Downing Street for bilateral talks with Prime Minister Sir Keir Starmer.
Qatar, one of the richest Gulf states, is a key mediator in the Israel-Hamas war, and there are reports the Government is hoping to seal a free-trade deal with the Gulf Co-operation Council.
But the country has faced criticism over its human rights record, in particular the abuse of migrant workers during Fifa World Cup in 2022, its criminalisation of same-sex relationships, and its discrimination against women.
Human rights campaigner Peter Tatchell condemned the decision to 鈥渞eward鈥 the Emir with the honour of a state visit, saying 鈥淔eting the Emir is collusion with tyranny鈥.
His Peter Tatchell Foundation is to stage a peaceful protest near Buckingham Palace, at Canada Gate to the right of the royal residence, on Tuesday morning ready for when Sheikh Tamim arrives in the carriage procession.
Mr Tatchell said: 鈥淜eir Starmer and King Charles should not reward the Emir with the honour of a state visit while his regime continues to victimise women, LGBTs and migrant workers.
鈥淔eting the Emir is collusion with tyranny.鈥
He added: 鈥淭he Qatar government is a police state dictatorship. It鈥檚 guilty of systemic homophobia, sexism and the suppression of workers鈥 rights and basic freedoms like free speech and the right to protest.
鈥淭his state visit sends the wrong message 鈥 that the UK prioritises trade and investment over human rights.
鈥淭he UK should be challenging Qatar鈥檚 human rights record and seeking the release of political prisoners, not rewarding its ruler with royal pageantry and red carpets.鈥
Mr Tatchell travelled to Qatar to stage an LGBT+ rights protest during the Fifa World Cup, but told how he was detained and interrogated by the authorities before being ordered to the airport to depart the country.
鈥淭his London protest is about continuing to expose the brutal reality behind Qatar鈥檚 carefully crafted public image,鈥 he said.
He added: 鈥淲e urge everyone who values equality and freedom to join our peaceful protest.鈥