The Prime Minister has agreed to two Brexit extensions, with the most recent taking the UK’s planned departure date to October 31. Mrs May hopes to use the time to persuade MPs to back her EU exit deal, or an alternative agreement drawn up with the opposition Labour Party. As part of the Government’s agreement with Brussels the UK will hold European Parliament elections in May.
Writing in The Sunday Telegraph Sir William commented: “After hours of discussions with QCs and former judges, I believe the British Government’s extension of Article 50 is unlawful.
“For the Prime Minister to agree to such an extension in these circumstances is to knowingly use her power in a way that she herself believes would risk frustrating Parliament’s intention that the UK must leave the EU.
“This is legally beyond the pale.”
Sir William argued the Government is not permitted to use its powers to frustrate the will of Parliament.
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Parliament approved legislation allowing the triggering of Article 50, the official EU exit mechanism, leading to the Government selecting March 29 as the official Brexit day.
He claimed this has not been overturned by Parliament, as it merely passed a resolution against a no-deal Brexit, and thus the law requires Britain to leave the EU.
Sir William asserted: “There are manifest limitations on the Prime Minister’s competence.
“They concern rules of UK internal constitutional law of fundamental importance.
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“In these circumstances, I believe it will be impossible for the Prime Minister, acting lawfully under UK law, to accept an extension of the kind proposed.”
Last week Sir William called on Mrs May to step down as Prime Minister in the House of Commons.
He also suggested the Government could face legal action over the Brexit delay in his article.
The veteran Eurosceptic argued: “A challenge in the courts is fully justified in respect of the purported extension time.”
The Government insists its decision to extend Article 50 and delay Brexit fully complies with UK law.