The official launch of Facebook's new cryptocurrency alliance was overshadowed on Monday by the abrupt departure of Booking.com, leaving the project with just 21 of its original 28 backers.

Booking Holdings, which controls 41pc of the online travel market through services including Kayak and Agoda, confirmed on Monday that it had dropped out on the very same day that it was supposed officially sign up to the project at a meeting in Geneva.

It was the seventh company to exit in the space of one week, joining Paypal, Mastercard, Visa, eBay and others in ducking out amidst unexpected levels of political hostility and regulatory scrutiny.

It came as Facebook attempted to shore up the currency's new governing body, the Libra Association, with companies including Uber, Lyft, Coinbase, Spotify and a number of development charities formally signing on in Switzerland as planned.

"Today marks an important step in our journey to enable billions of people to have access to essential financial services and the world's economy," said a spokesperson for Calibra, Facebook's cryptocurrency subsudiary.

"We are proud to join the Libra Association as collectively we seek to increase innovation and economic empowerment. It is time for change and we are committed to seeing this through."