WASHINGTON (Reuters) - The European Union on Wednesday called on President Barack Obama to block a U.S. bill allowing survivors and families of victims of the Sept. 11 attacks to sue Saudi Arabia, saying it was in violation of international law.

"The possible adoption and implementation of the (bill) would be in conflict with fundamental principles of international law and in particular the principle of State sovereign immunity," the European Union delegation to the United States said in a letter to the U.S. Department of State seen by Reuters.

The letter said the EU considers that the bill's adoption and its implementation could have unwanted consequences, as other states adopt similar legislation.

Congress has overwhelmingly passed the Justice Against Sponsors of Terrorism Act, known as JASTA, in reaction to long-running suspicions, denied by Saudi Arabia, that hijackers of the four U.S. jetliners that attacked the United States in 2001 were backed by the Saudi government.

Obama in coming days is expected to veto the bill on grounds that other countries could use the law as an excuse to sue U.S. diplomats, service members or companies.

But Congress could have the last word if the Senate and House of Representatives each override that veto by a two-thirds vote.

(Reporting by Patricia Zengerle; Editing by Andrew Hay)