Organisers will on Tuesday announce the host of the 2030 World Expo, with Rome in Italy, the Saudi capital Riyadh and South Korea's Busan contending to welcome the showpiece event held every five years.

The World Expo, which traces its history back to the 1851 Great Exhibition in London, is a months-long mega event attracting millions of visitors that aims to respond to the specific challenges of the current time.

The hosting of this year's event has been the subject of particularly intense lobbying from Italy, Saudi Arabia and South Korea, whose governments are all keen to have the honour of the 2030 World Expo.

Saudi Crown Prince Mohammad bin Salman has been at the forefront of the kingdom's efforts to host the Expo, which would be the latest in a line of events to be hosted by the kingdom despite concern over its rights record.

It is set to host the 2034 football World Cup after emerging as the only bidder for the event.

But Rome and Seoul have also not stinted in their efforts.

South Korean President Yoon Suk Yeol was in the French capital only last week to make a final pitch for Busan's bid, while Italy's right-wing Premier Georgia Meloni has pressed Rome's cause in Paris and elsewhere.

- 'History of rights violations' -

Awarding the Expo to Saudi Arabia could prove controversial.

But France's ambassador to Riyadh Ludovic Pouille, writing on X, formerly Twitter, reaffirmed that France "supports the Saudi capital's bid" and noted that Paris was already "decked out in Riyadh's colours" with billboards ahead of the announcements.

President Emmanuel Macron had already given Riyadh France's backing when he hosted the crown prince for talks at the Elysee in July 2022, a move that caused some irritation for France's Italian EU partners.

- Russia, Ukraine out -

The hosting of the Expo is regulated by the Paris-based Bureau International des Expositions (BIE), an organisation created almost a century ago which now has 182 member states.

The host is chosen at the BIE's general assembly on Tuesday with the candidates making a final presentation and then the winner chosen in rounds of voting by the member states in the afternoon.

Every member state -- big or small -- has a single vote, meaning a vast global campaign is needed to ensure victory.

Busan wants to hold the Expo from May-October 2030 under the theme "Transforming Our World, Navigating Toward a Better Future". Rome plans the Expo in the same period to be about "People and Territories: Regeneration, Inclusion and Innovation".

A Riyadh Expo would be from October 2030 to March 2031 under the theme "The Era of Change: Together for a Foresighted Tomorrow".

The most recent World Expo took place in 2020 in Dubai and the next World Expo is due in 2025 in Osaka, Japan under the theme "Designing Future Society for Our Lives".

Other bids for 2030 came from the Russian capital Moscow and the Ukrainian city of Odesa. But Russia withdrew its bid in May 2022 shortly after its invasion of Ukraine in February that year, while Odesa did not make the final shortlist of three announced in June.