Belgian authorities are advising against travel to Brussels following a series of terror attacks in the city.
Two explosions went off at Brussels international airport, killing 11 people, and there was a third blast in the carriage of a metro train at a station in the city centre, killing 20. Dozens more were injured, some critically.
Zaventem Airport was badly damaged and remains closed until further notice.
All flights have been suspended until Tuesday morning. However, flights continue to the city's other airport, Charleroi.
Ryanair said services due to operating to Zaventem on Wednesday will be re-routed to Charleroi.
Eurostar services were suspended to Brussels following the blasts but a limited service began operating later in the day.
Normal Eurostar services are due to resume on Wednesday and public transport in the city has re-opened.
Although March is not a busy time for leisure tourism, Brussels is a year-round destination for business travel.
The Guild of Travel Management Companies said the events in Brussels demonstrate the importance of putting systems in place to protect travellers.
"Unfortunately planning around horrible events has become commonplace for the professional travel management sector," it said.
"The increasing advent of major travel disruption and extensive questions about the safety and security of those travelling for business is something that TMCs are prepared for.
"Duty of care is exceptionally important, but as is being able to conduct business safely during difficult scenarios. Everyone in the business travel industry hopes that these skills, and the supporting technology and booking processes, are never needed.
"Corporate travel planners should be checking and re-checking their approaches, policies and reporting in order to understand what they would do if impacted by what has unfolded in Brussels, and ensuring that they are confident their people can be safely tracked. Sadly, these are essential planning processes for any business with people who travel."
Colleen Lerro Gallagher, communications director for the GBTA, said business travel must continue despite terror attacks.
"GBTA's Risk Committee has previously spoken about understanding the difference between living in terror and living with terror because the reality is people still need to travel and businesses still have to do business," she said.
"We live in a global economy that still requires face-to-face interaction to move forward. It's important for travel buyers to keep informed so they can make prudent decisions for their travel programme."
Belgium has raised its terror alert to the highest level, which means an attack is believed imminent.