Prayut, a former army chief who seized power from Thailand's civilian government in a 2014 coup, will next meet French President Emmanuel Macron on Monday as part of a highly-publicised European tour.

"The Prime Minister urged continued progress towards free and open elections in Thailand in line with international standards, including restrictions on political parties being lifted at an early stage," her spokesperson said after the meeting.

May's meeting with Prayut came just two days after Thailand carried out its first execution since 2009, prompting outrage from human rights groups.

During their sit-down, Prayut assured May that Thailand would hold a general election early next year.

Prayut, now prime minister, is bidding to boost his image as a politician rather than military man as many Thais weary of junta rule ahead of long-delayed elections promised for February.