UNITED NATIONS, N.Y., Oct. 14--Britain today accused the Soviet Union of confronting the U.N. with an ultimatum by threatening to go ahead with nuclear weapons tests unless the Western powers agree to Soviet proposals.

Soviet Deputy Foreign Minister Valerian A. Zorin immediately denied the ultimatum charge. He called on the U.N. to approve a Soviet-proposed recommendation that the Big Three suspend tests immediately and unconditionally for all time.

The United States and Britain have announced they will suspend tests for a year beginning Oct. 31, provided the Russians do the same.

Ike Urges GOP On

WASHINGTON, Oct. 14--President Eisenhower, in a peppery mood at his 68th birthday party, urged Republicans today to come out slugging against the Democrats in the elections campaign.

In obvious reference to the GOP's defeat in Maine's September election, Eisenhower said that there is no reason for the Republicans to be discouraged.

He said that his administration had given the country "sane, sound, logical government as opposed to radical government."