Theresa May's Brexit manifesto promises could be rejected by the Lords as pro-European Union figures said her failure to win a majority means they are no longer protected.

The Telegraph understands Labour and Liberal Democrat peers believe the Salisbury Convention, which normally protects manifesto pledges, does not apply because the Prime Minister has failed to win a majority.

It casts huge doubt on Mrs May's ability to deliver on Brexit because the Lords could refuse to wave through major legal changes and prevent efforts to take Britain out of the single market.