* Shield initially scheduled to be operational in 2018
* Polish government and U.S. say delay due to technical reasons
* Critics say delay is sign of deteriorating ties (Adds name of construction company)
By Marcin Goclowski and Lidia Kelly
WARSAW, March 22 (Reuters) - Technical problems will delay the completion of the Polish section of a U.S. defence shield by two years to 2020, Poland's defence ministry said on Thursday.
The anti-missile site close to Poland's northern Baltic Sea coast is part of a NATO defensive umbrella that, when complete, will stretch from Greenland to the Azores.
Poland's Defence Ministry said U.S. authorities had told it there had been construction delays at the Redzikowo site that were the responsibility of the contractor.
The U.S. head of the U.S. Missile Defence Agency (MDA), Lieutenant General Samuel Greaves, acknowledged the delay at a U.S. Senate hearing on Thursday.
The MDA, a unit of the U.S. Defence Department, is overseeing installation of the Lockheed Martin Co -made ground-based Aegis ballistic missile defence system.
"While we have experienced delays in the military construction portion of the Aegis Ashore effort in Poland, we remain steadfastly committed to delivery of that capability," Greaves said.
According to U.S. Department of Defense records, AMEC Programs Inc of Alpharetta, Georgia, was awarded a contract worth $182 million for military construction, research, development, testing, and evaluation for the Aegis site in Poland.
Calls to AMEC were not returned.
Officials from the U.S.-led NATO alliance, who took control of the umbrella in 2016, have said the whole system is supposed to defend against attacks by states such as Iran and groups such as al Qaeda.
But the scheme has angered Russia, incensed by the show of force by its old Cold War rival in formerly communist Eastern Europe.
Poland's former defence minister, Tomasz Siemoniak, from the opposition Civic Platform (PO) party, said the delay was meant as a political rebuke to the current government.
"This is the price for the ... lack of trust in PiS (the ruling conservative Law and Justice party)," Siemoniak told Reuters.
Washington sharply criticised a bill that Poland adopted in January that imposes jail terms for suggesting Poland was complicit in the Holocaust, saying it was concerned about the repercussions it could have "on Poland's strategic interests and relationships".
Siemoniak said that "in the context of the (Holocaust bill), the lack of trust has deepened ... 'Technical reasons' are just a pretext to postpone it until the year 2020. I am convinced that the situation is a result of the deterioration of the allied relations."
Bartosz Cichocki, a deputy foreign minister, told the PAP state news agency that there was no crisis in Polish-American relations and that the U.S. had informed Poland about the potential delay "weeks before" the Holocaust law was adopted.
As part of the shield, the United States switched on an $800 million site in Romania in May 2016 and broke ground at the site in Poland later that year.
The full shield also includes ships and radar installations across Europe, with command and control run from a U.S. air base in Germany. (Additional reporting by Mike Stone in Washington and Robin Emmott in Brussels; writing by Lidia Kelly; editing by Matthew Mpoke Bigg and Cynthia Osterman)