Simone White, 28, a lawyer from southeast London, has been identified by British media as among the tourists who died after consuming drinks suspected to have been laced with methanol.
"We are supporting the family of a British woman who has died in Laos, and we are in contact with the local authorities," Britain's foreign ministry said in a statement.
An Australian, two Danes and an American have also died in the apparent mass poisoning in Vang Vieng, a popular tourist town in the landlocked Southeast Asian nation.
Melbourne teenager Bianca Jones, 19, was confirmed to have died by her family on Thursday, sparking a wave of emotional tributes from loved ones and political leaders.
Her friend Holly Bowles, also 19, continues to fight for her life in a different Bangkok hospital.
The pair, from Beaumaris in Melbourne's southeast, had been holidaying in Laos when they became ill along with a dozen other tourists in Vang Vieng last week.
They had planned to stay at the Nana Backpacker Hostel in Vang Vieng and drank at the venue before reportedly going to Jaidee Bar on November 11.
Ms Jones and Ms Bowles did not leave their dorm room for 24 hours and reported feeling unwell on November 13, before being taken to separate hospitals in Bangkok.
Shaun Bowles told reporters outside Bangkok Hospital on Wednesday that his daughter remained in critical condition and on life support.
"We just like to thank everyone from back home for all of the support and love that we're receiving," he said.
"But we'd also like the people to appreciate right now, we just need privacy so we can spend as much time as we can with Holly."
Consular assistance is being provided to the families of the two Australians girls.
Hostel manager Duong Duc Toan said the Australian women had joined more than 100 other guests for free shots of Lao vodka offered by the hostel as a gesture of hospitality.
Thai authorities confirmed Ms Jones had died by "brain swelling due to high levels of methanol found in her system".
The Australian government has updated its travel advice for Laos, telling residents to be alert to the potential risk of spirit-based drinks including cocktails.
Foreign Minister Penny Wong said drink-spiking and methanol poisoning were "far too common in many parts of the world".
"At this time I would say to parents, to young people, please have a conversation about risks, please inform yourselves, please let's work together to ensure this tragedy does not happen again," she said.
Ms White's friend Bethany Clarke also urged tourists in the region to avoid local spirits.
"Just avoid them as so not worth it," she wrote in a Laos backpacking Facebook group.
"Six of us who drank from the same place are in hospital currently with methanol poisoning."
One New Zealand citizen has also fallen ill in Laos and could be a victim of methanol poisoning, New Zealand's Foreign Ministry said.
"Travellers are advised to be cautious about consuming alcoholic beverages, particularly cocktails and drinks made with spirits that may have been adulterated with harmful substances," it said.
With Reuters and AP
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