Mexico's government on Monday outlined the scheme due to be implemented on June 1, and Cabify, Uber Technologies and several other companies said they would participate.

Chinese rival Didi said it would not take part to begin with, however, prompting criticism from Cabify and others that their absence from the programme could distort the market.

"What's been discussed with the finance ministry is that this year it'll be a voluntary programme ... and it's very likely that next year it'll be part of ... the budget spending plan or a new law," Ramon Escobar, Cabify's director in Mexico, told reporters on a conference call.

When asked if that meant it would become binding, he said "that's the conversation we're having today."

Mexico's finance ministry did not immediately respond to a request for comment.

A Didi spokesman said the company will "continue to comply with all tax laws and obligations, and we will continue to encourage our driving partners to do the same." The firm would continue to analyse its participation in the scheme, he added.

The scheme foresees a monthly value-added tax (VAT)withholding rate of 8% and an income tax rate ranging from 3% to 9%, according to Uber.

(Reporting by Sharay Angulo and Julia Love; Editing by Sonya Hepinstall)