Two law professors in Turkey have written to Twitter asking the social network to cease blocking accounts in the country because it violates human rights.
Yaman Akdeniz and Kerem Altıparmak said that if Twitter continues to block accounts, the pair will "take all possible forms of legal action in both Turkey and the United States".
In March 2014, Turkish authorities blocked access to Twitter. Then Prime Minister Recep Tayyip Erdogan - now Turkey's president - vowed to "wipe out" the social networking site.
The ban was lifted two weeks later, but since then many accounts tweeting posts hostile to the government have been blocked.
One whistleblowing Twitter user, @fuatavni, has been a particular scourge to the government.
In December, the account reported on corruption raids against ministers and mayors.
On 5 August the account was blocked in Turkey following a court order. Fuat Avni has not been identified, but this week his or her social media accounts on Twitter and Facebook were again blocked by a court order.
The public prosecutor’s office has launched a formal investigation into "alleged illegal wiretapping" by Fuat Avni.
Other Turkish social media users have also fallen foul of a regime intent on policing expression on social media.
Journalist Sedef Kabas will face trial, after she tweeted reminding her users of the name of a judge who dropped a corruption probe.
Kabas told a Turkish news site that she was charged with "targeting people who are involved in anti-terror operations."
The professors argue in the formal legal notice that Twitter's blocking of accounts has "turned into clear violations of the freedom of the press."
They say that Twitter, as a US company, has no legal obligation to Turkish courts, but, as a non-state actor, does have obligations to respect "all human rights in general and freedom of expression in particular."
The professors also said: "The procedure followed in the issuance of blocking orders and the content of the decisions are explicitly against international human rights law ... and the Constitution of the Republic of Turkey."
Sky News has asked Twitter for a response.