As of September, the Singapore Tourism Board (STB) had revoked the licences of three travel agencies.
On Tuesday, a fourth travel agent, Sky Travel & Tours, was told it would lose its licence after it shuttered its doors on Monday, leaving customers in the lurch.
In a statement on Tuesday, STB said a Notice of Revocation of Travel Agency Licence had been served on the company, which had ceased operations and was unable to fulfil its obligations to its customers.
Among them was Madam Teo Ino Meo, 68, a cleaner who had paid $2,400 for a week-long tour to Hokkaido, but learnt hours before her flight on Monday that the tour would not proceed.
Yesterday, the Consumers Association of Singapore (Case) said it had received 11 complaints against Sky Travel since Monday.
STB is also looking into the matter and may consider taking further action against Sky Travel and its directors, if necessary.
It said affected customers should contact the agency for the status of their booking or to seek a refund, or their travel insurance providers if that cannot be resolved.
An STB spokesman said many affected Sky Travel customers had purchased travel insurance, which protects against unforeseen circumstances such as this.
Madam Teo said she had travel insurance, but did not know if it covered the agent's insolvency.
Travel agents stressed the importance of insurance, especially for protection against agents becoming insolvent.
An STB licensing condition, in effect since last year, requires agents to remind customers to buy travel insurance, including that against travel agent insolvency, said Dynasty Travel director of marketing communications Alicia Seah.
"We abide by this regulation because we believe that customers should have peace of mind when booking holidays," she said.
Purchasing such insurance also means that customers can get back at least a portion of their payment, she added.
A Chan Brothers Travel spokesman said the regulation also benefits the travel industry as it safeguards the reputation of the tourism sector.
She added: "However, this regulation does not address the root of the problem entirely and more measures could be put in place to protect consumers, such as a higher set-up capital or STB reviewing the criteria in issuing travel agent licences."
An STB spokesman said three travel agency licences were revoked last year, and four in 2014, but not all were due to agencies closing down.
Case executive director Seah Seng Choon said 548 complaints were made against the travel industry as of last month. There were 1,037 last year and 1,926 in 2014.