European privacy watchdogs have issued a warning to WhatsApp over a recent policy change which allows the messaging company to share phone numbers with its parent company Facebook.

A grouping of the EUs 28 data protection authorities – known as the Article 29 Data Protection Working Party – has sent a letter to the popular messaging service expressing “serious concerns regarding the sharing of information within the ‘Facebook family of companies.’”

In the letter, the group have requested that WhatsApp stop sharing users' data with Facebook until the "appropriate legal protections could be assured" to avoid falling foul of EU data protection law.

A statement from the group said the policy change - the first since WhatsApp was acquired by Facebook in 2014 - may involve the sharing of information for purposes that were not included in the terms of service when users signed up. 

The group said the issue raises concerns about the validity of users consent, the mechanisms provided to users to exercise their rights and the rights of non-Facebook users.

The Italian antitrust watchdog on Friday also announced a separate probe into whether WhatsApp obliged users to agree to share personal data with Facebook.

The Italian agency said it was investigating whether WhatsApp had led users to believe they would be unable to continue using the service unless they agreed to the new terms and conditions.

It is also looking into certain other elements of the terms of use - including potential “unjustified” interruptions to service and a regulation that means only WhatsApp has the right to terminate the agreement.

The maximum fine that could be levied at the end of the investigation is €5m.

Earlier this year WhatsApp justified the changes by saying the new policy would allow for “more relevant” suggestions regarding who people should connect with.

The company had previously pledged to remain independent of Facebook.

A spokeswoman for WhatsApp today told Reuters news agency the company was working with data protection authorities to address their questions – and is committed to respecting the law.