The letter, which is signed off by ‘The Real 48 Per Cent’, appears to be anti-Brexit, stating: “If you attempt to take away part of someone’s identity, there are consequences.
“We have watched you as you have led us to the edge of the abyss. We will watch no longer.
“You have taken lives on our side. Now we will take lives on yours. We are coming for you.”
The space normally reserved for an address instead reads: “We were born in Britain. We live in the UK. We are European.”
Ms Leadsom condemned the writer’s actions, saying on Twitter alongside a photo of the letter: “Pretty despicable whoever sent me this. We live in a democracy -death threats because you don’t agree? And unsigned? coward...”
A spokesman for the Metropolitan Police said: “Police are aware of threatening letters sent to four Members of Parliament at the House of Commons.
“Two of the letters were reported to police on Friday, February 9, and two on Monday, February 12.
“The letters are being investigated by officers from the Parliamentary Liaison and Investigation Team within the Parliamentary and Diplomatic Protection Command.
“There has been no arrest at this stage. Inquiries are ongoing.”