The Salisbury Convention states that manifesto commitments made by a governing party should not be blocked or significantly altered by the Lords.

However, within hours of Wednesday’s Queen Speech, which set out the various Brexit bills to be presented before Parliament in the coming months – opposition parties began to argue that the convention does not apply as Mrs May did not win a majority and has not agreed to govern in coalition.

The legitimacy of the convention is set to spark a major constitutional row between legal experts.

Meanwhile, the Scottish National Party (SNP) is demanding that a separate vote be held in Scotland to determine whether Mrs May can repeal EU laws – a crucial step in the Brexit process.

Downing Street said last night that the legal role of the Scottish Parliament was also still being considered. The growing constitutional row over Brexit has added to the pressure on the Prime Minister to secure a formal deal with the Democratic Unionist Party (DUP) that would add to the legitimacy of her Government.