Twitter announced Sunday it would no longer allow users to promote their accounts on several rival social media platforms including Facebook and Instagram, but the site's mercurial owner Elon Musk appeared to backtrack on the new policy just hours later.

The sudden shift in the rules was the latest in a series of controversial changes made by Musk since he took over the company in October -- upheaval that has led a growing number of users to encourage followers to view their posts on other sites.

The unpredictable billionaire even put his future as Twitter's CEO to a vote.

"Should I step down as head of Twitter?" he tweeted, asking the site's users to click yes or no.

"I will abide by the results of this poll," he added, with the vote open until the early hours of Monday.

Twitter had announced that the company would "no longer allow free promotion of specific social media platforms."

"At both the Tweet level and the account level, we will remove any free promotion of prohibited 3rd-party social media platforms, such as linking out (i.e. using URLs) to any of the below platforms on Twitter, or providing your handle without a URL," the company explained in a statement.

Users would thus be barred, for example, from posting "Follow me @username on Instagram," Twitter said.

Twitter co-founder Jack Dorsey questioned the new policy with a one-word tweet: "Why?"

After some notable accounts were suspended under the new policy, including tech investor Paul Graham, Musk tweeted that instead of considering individual tweets, the policy would be limited to "suspending accounts only when that account’s *primary* purpose is promotion of competitors."

He later said: "Going forward, there will be a vote for major policy changes. My apologies. Won't happen again."

- Changes under Musk -

The move was the latest in a growing series of controversies generated by Musk in his short tenure at the helm of Twitter, including layoffs, reinstatement of some far-right accounts and the suspension of several journalists.

Shortly after taking over the platform, he announced the site would charge $8 per month to verify account holders' identities, but had to suspend the "Twitter Blue" plan after an embarrassing rash of fake accounts. It has since been relaunched.

On November 4, with Musk saying the company was losing $4 million a day, Twitter laid off half its 7,500-strong staff.

Musk also reinstated the account of former president Donald Trump and said Twitter would no longer work to combat Covid-19 disinformation.

In recent days, he suspended the accounts of several journalists -- most recently, Washington Post reporter Taylor Lorenz -- after complaining some had divulged details about the movements of his private jet that could endanger his family.

The suspension of the journalists -- employees of CNN, The New York Times and The Washington Post were among those affected -- has drawn sharp criticism, including from the European Union and the United Nations.

The US Federal Trade Commission said it was tracking developments at Twitter "with deep concern."

Washington Post executive editor Sally Buzbee said the suspension of Lorenz's account "further undermines Elon Musk's claim that he intends to run Twitter as a platform dedicated to free speech."

Some of the suspended accounts have since been reactivated.