Greece is in the middle of tough talks with auditors from the EU, the European Central Bank and the International Monetary Fund.
"An array of achievements under our common European acquis have been (curtailed), chief of them the acquis of social rights, specifically...labour rights and collective bargaining," Tsipras said in the letter to EU leaders seen by AFP.
"In my country, this right is restricted."
"I ask for your support to jointly defend Greece's right to return to European social model standards," the PM said in the letter also sent to EU Commission President Jean-Claude Juncker and EU President Donald Tusk.
The EU's 27 leaders on Saturday will celebrate the 60th anniversary of the Treaty of Rome which gave birth to the European Economic Community, the bloc's precursor.
There has been speculation in Athens that Tsipras would not sign the common declaration to be adopted during the festivities, unless it included specific references to the so-called social acquis, social achievements.
"It is our intention to support the declaration," Tsipras said in his letter.
"But in order to truly celebrate these achievements, we must know openly, officially and clearly, if we too are entitled to have access to them," he added.
Greece is in the middle of tough talks with auditors from the EU, the European Central Bank and the International Monetary Fund on fresh cuts required to unblock promised loans.
Among the measures reportedly demanded by the creditors are new pension cuts and more restrictive rules to limit union strikes.