Lockheed Martin Corp (LMT.N), the largest U.S. weapons maker, has agreed to buy United Technologies Corp's (UTX.N) Sikorsky Aircraft unit, the maker of Black Hawk helicopters, for over $8 billion, a source familiar with the negotiations said on Sunday.

The two companies plan to announce the deal on Monday, said the source, who was not authorized to speak publicly.

It will be Lockheed's largest acquisition since it bought Martin Marietta Corp for about $10 billion two decades ago, and the first major strategic move for both United Tech Chief Executive Officer Greg Hayes, who was elevated to CEO from finance chief in November, and Lockheed CEO Marillyn Hewson, who took over her job in January 2013.

Officials at United Technologies declined comment. No comment was immediately available from Lockheed.

(Reporting by Andrea Shalal; Editing by Andrew Heavens)