Labour peer Lord Falconer, who previously served as Tony Blair’s Lord Chancellor and Justice Secretary, also accused Mrs May of being “ignorant” of EU law.
The pro-EU campaigner said: “That is so ignorant, so illiberal, so misguided.
“Ignorant because you have to be a member of the ECHR to be a member of the EU. The EU itself agrees to abide to the ECHR.
“Illiberal because there has to be a source external to a government determining what human rights are.
“And misguided because it will so damage the standing of the UK – the country that above all plays by the rules and that is going around the world saying we should comply as a world with human rights.”
Labour’s shadow home secretary Andy Burnham said leaving the ECHR would be a “backward step”.
He added: “What a terrible message to send to the world. Will fight May’s plans all the way.”