The troubles faced by co-working titan WeWork are darkening the outlook for the world's largest business hubs, where rising office vacancies are already heaping pressure on investors set to refinance big-ticket mortgages next year.

Media reports on Wednesday suggested the New York-listed flexible workspace provider — once privately valued at $47 billion — was weighing a petition for bankruptcy next week.

Backed by Japan's SoftBank, WeWork aimed to revolutionize the office market by taking long leases on large properties and renting the space to multiple smaller businesses on more flexible, shorter arrangements.