; and (B) approved by Congress under section 101(a)(1) of the United States-Mexico-Canada Agreement Implementation Act ( 19 U.S.C. 4511(a) ). (2) United Kingdom The term United Kingdom means the United Kingdom of Great Britain and Northern Ireland. 4. Negotiating and trade agreements authority for comprehensive agreement with the United Kingdom (a) Initiation of negotiations Not later than 180 days after the date of the enactment of this Act, in order to enhance the economic well-being of the United States, the President shall seek to initiate negotiations with the United Kingdom regarding tariff and nontariff barriers affecting any industry, product, or service sector. (b) Authority for comprehensive trade agreement with the United Kingdom (1) In general To strengthen the economic competitiveness of the United States, the President may enter into a comprehensive trade agreement with the United Kingdom regarding tariff and nontariff barriers affecting trade between the United States and United Kingdom. (2) Termination of authority The authority under paragraph (1) terminates on March 1, 2025. (c) Modifications permitted (1) In general Subject to paragraph (2), the President may proclaim such modification or continuance of any existing duty, continuance of existing duty-free or excise treatment, or such additional duties as the President determines to be required or appropriate to carry out an agreement entered into under subsection (b). (2) Limitations (A) Modifications or additions to agreement Substantial modifications to, or substantial additional provisions of, an agreement entered into after March 1, 2025, are not covered by the authority under paragraph (1). (B) Amount of duty modification No proclamation may be made under paragraph (1) that— (i) reduces any rate of duty (other than a rate of duty that does not exceed 5 percent ad valorem on the date of the enactment of this Act) to a rate of duty that is less than 50 percent of the rate of such duty that applies on such date of enactment