Unionized nurses have filed an unfair labor practice complaint with the National Labor Relations Board charging that UMass Memorial Medical Center in Worcester is breaching its contract and violating federal labor law by requiring some nurses to work on Christmas.

Between 10 and 20 nurses were told in recent days that they will be needed on Christmas even though they worked on Thanksgiving and are scheduled to work New Year’s Day, said David Schildmeier, spokesman for the Massachusetts Nurses Association, which filed the complaint. Under their contract with UMass Memorial, he said, nurses are not required to work on two consecutive holidays.