denounce

To make known in a formal manner; to proclaim; to announce; to declare.

Nero[…]sent his Satellites or officers toward him, to denounce the decree of his death to him[…].

And full of peace, denouncing wrath to come

To criticize or speak out against (someone or something); to point out as deserving of reprehension, etc.; to openly accuse or condemn in a threatening manner; to invoke censure upon; to stigmatize; to blame.

to denounce someone as a swindler, or as a coward

Mr. Cameron had a respite Thursday from the negative chatter swirling around him when he appeared outside 10 Downing Street to denounce the murder a day before of a British soldier on a London street.

To make a formal or public accusation against; to inform against; to accuse.

to denounce a confederate in crime

to denounce someone to the authorities

To proclaim in a threatening manner; to threaten by some outward sign or expression; make a menace of.

to denounce war; to denounce punishment

To announce the termination of; especially a treaty or armistice.

It would be possible to “denounce” (leave) the Convention altogether, but short of that, legislation to restore the death penalty would place the UK government in breach of its treaty obligations under the ECHR; it would breach international law.

A Contracting State may denounce this Convention, or Part II or Part III of the Convention, by a formal notification in writing addressed to the depositary.