A team of international toxicologists has been called in to investigate the sudden deaths of five foreign tourists in the Thai city of Chiang Mai, after doubts were cast over claims made by local authorities that the deaths were coincidental, and potentially caused by food poisoning.

Thailand's public health department said yesterday that it had enlisted experts from the UK, the World Health Organisation, and US Centers for Disease Control and Prevention among others, "to determine whether toxins or chemical agents might be involved in the deaths". It is now thought that pest control products could be to blame.

George and Eileen Everitt from Lincolnshire, a 23-year-old New Zealander, Sarah Carter, and a Thai tour guide all died within 16 days in adjoining rooms at the three-star Downtown Inn in Chiang Mai in February. All four are believed to have died after the sudden onset of severe heart conditions.

Authorities are now also investigating the deaths of two other foreign tourists, who fell ill in Chiang Mai at a similar time with similar symptoms. The city's governor had described the deaths as a "coincidence". Thai health authorities had blamed food poisoning from "toxic seaweed" for Ms Carter's death.

Mr and Mrs Everitt's son, Stephen, said: "If insecticides are to blame, I want to know why such deadly chemicals would be used in a hotel."