The US Federal Trade Commission said Facebook and WhatsApp must stand by their previous commitments to protect user privacy.

Jessica Rich (pictured), director of the FTC’s bureau of consumer protection, told both companies they must stick to statements made in February when Facebook announced the $19 billion acquisition of WhatsApp.

“WhatsApp must continue to honour these promises to consumers,” she wrote.

However, if the acquisition is completed and WhatsApp does not stick to its guns, both companies could be in violation of Section 5 of the Federal Trade Commission Act and, potentially, an existing FTC order against Facebook.

In 2011, Facebook settled FTC charges that it had deceived users by failing to keep its own privacy promises.

Under the commission’s order, Facebook must get affirmative consent from users before making any changes that override their privacy settings, among other requirements.

Rich’s letter also said before making any changes to how  data already collected from WhatsApp customers is used, the companies must get their affirmative consent.

The letter notes that the companies must not misrepresent the extent to which they maintain the privacy or security of user data.

It also recommends that consumers be given the opportunity to opt out of any future changes to how newly-collected data is used.

Jan Koum, founder and CEO of WhatsApp, blogged last month about how central user privacy is to the messaging firm.

He pointed out that users aren’t required to give WhatsApp their name, email, date of birth, home address, where they work, what they like, their internet search information or GPS location.

“None of that data has ever been collected and stored by WhatsApp, and we really have no plans to change that,” he said.

How the company sticks to such commitments will likely come under keen observation in the coming months and years.

Leveraging user data for advertising of course is a key means to monetise WhatsApp’s service and recoup the $19 billion pricetag.

Facebook did not comment on the FTC’s concerns but said it was “pleased” the commission had completed its review and cleared the acquisition.