Ian Blackford has written to the Lord President of the Council, Jacob Rees-Mogg, to formally seek access to any meeting of the Queen’s advisers on the Brexit legislation to “object in the strongest possible terms” in person. It follows a vote in the Welsh Assembly yesterday which saw it refuse legislative consent for the WAB, following similar votes in the Scottish Parliament and Northern Irish Assembly.

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The Scottish Government has led protests from devolved administrations over claims of a “power grab” under Brexit legislation. Under the terms of the Sewel Convention, the UK government does not “normally” legislate in areas that are devolved, but ministers in London say doing so is essential to preserve the UK market after leaving the EU.

However, the Supreme Court has ruled that the convention does not have legal force and cannot prevent Westminster from legislating.

As leader of the third party in Parliament, Mr Blackford is a member of the Privy Council, which performs a crucial constitutional function despite being largely ceremonial.

In his letter to Mr Rees-Mogg, the SNP’s Westminster leader states: “The granting of Royal Assent without legislative consent would represent a serious breach of the letter and spirit of the Sewel Convention, which has underpinned devolution on these islands for the past 20 years. As a member of Her Majesty’s Privy Council, I am also formally requesting the right to attend any meeting of the council convened for the purpose of granting Royal Assent to the bill.

“I will wish to object in the strongest possible terms at the meeting to Assent being given to a bill without consent from the devolved legislatures, and which enacts a decision to leave the European Union overwhelmingly rejected by voters in Scotland in the 2016 referendum.”

Mr Blackford accused ministers of “breaching the devolution settlement and trampling over democracy”.

It came as Downing Street pledged to strip away all amendments to the WAB after peers inflicted five defeats on the government over the legislation moved through the House of Lords.

On Monday, peers voted to force the government to issue physical proof of settled status for EU citizens living in the UK.

Yesterday, the Lords backed a move to ensure the rights of unaccompanied child refugees to be reunited with their families in the UK after Brexit.

SNP mounts devolution protest at Privy Council over Brexit bill