King Charles and Keir Starmer welcomed Qatar's emir, Sheikh Tamim bin Hamad Al Thani, for a state visit to Britain on Tuesday that the prime minister hopes will help build on the Gulf state's investment in the country.

The emir and wife Sheikha Jawaher bint Hamad bin Suhaim Al Thani arrived by car at Horse Guards Parade in London with Prince William and his wife Kate, who was marking her return to formal state visit duties after undergoing preventative chemotherapy for cancer.

Charles, who is continuing his own treatment for cancer, and the emir inspected the 1st Battalion Welsh Guards while a military band played.

The morning of pageantry was to be followed by a trip to Westminster where the emir will address both chambers of the Houses of Parliament.

Britain is seeking deeper ties with the wealthy Gulf state, and Starmer hopes to use the visit to secure "tangible benefits" for the country on security and the economy, his spokesperson said.

Starmer was elected in July on a promise to lift economic growth and is looking to potential wealthy investors such as Qatar to help fund his plans for new infrastructure and energy developments.

Qatar is already a big investor in Britain through the Qatar Investment Authority which owns the Canary Wharf business and entertainment district in east London, the Shard landmark skyscraper in central London, and stakes in bluechip names such as Barclays and Heathrow Airport.

"You can expect them to discuss how we can build further on our two countries' thriving trade and investment partnership," the prime minister's spokesperson said.

The two royal families departed by horse-drawn carriage for Buckingham Palace where they were due to have lunch with Queen Camilla, who was absent from the outdoor event as she continues to recover from a chest infection. (Reuters)