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(This May 21 story corrects contract expiration to April 29 from almost a year ago)

(Reuters) - Westinghouse Electric Co, a unit of Japan's Toshiba Corp 6502.T, said on Sunday it issued a lockout notice to 172 union members at its nuclear components manufacturing plant in Newington, New Hampshire, declaring that the sides had reached a stalemate in contract negotiations.

Westinghouse, which has been operating under bankruptcy protection, had only begun formal negotiations with the International Brotherhood of Boilermakers, Iron Shipbuilders, Blacksmiths, Forgers and Helpers in April for a contract that expired April 29 of this year.

The Boilermakers and Westinghouse engaged in a mediation process that was unfortunately not successful, and the union has refused to accept the company’s last, best and final contract offer, the company said in a statement.