The Army and police are investigating after a soldier sent an offensive tweet to Labour MP Angela Rayner.
The soldier’s Twitter account – which has now been deleted – posted a message swearing at Ms Rayner and saying she “will perish when civil war comes”.
Ms Rayner called it an example of a “usual vile tweet I get daily”.
Defence Secretary Ben Wallace said it was “unacceptable”, while a senior Army commander apologised and said it “is being dealt with”.
It comes as MPs have spoken more frankly about the abuse they face amid a row over the language used by politicians.
Shadow education secretary Ms Rayner, the MP for Ashton-under-Lyne in Greater Manchester, took a screenshot of the soldier’s tweet, which was sent in reply to one of her tweets criticising the government’s Attorney General Geoffrey Cox.
In the tweet, the soldier said: “17.4 million people are gunning for blood if we don’t leave”.
Ms Rayner shared the screenshot and later tweeted: “I have constantly called out abuse publicly against elected MPs including to Tories as I think our democracy is built on mutual respect for difference and the rule of law.
“What’s being unleashed is ugly and should be condemned by all.”
On Friday, Mr Wallace said: “This is an unacceptable tweet from a member of the British Army to Angela Rayner.
“This foul language goes against the values of the Armed Forces and is now being investigated by the Army and civilian police.”
Lt Gen Ivan Jones, who is commander of the Field Army, tweeted that he would like to “apologise personally” to Ms Rayner and “anyone affected by appalling tweets” from the British soldier.
“He does not represent the remarkable men and women in [the] British Army who serve this nation,” Gen Jones wrote.
“Rest assured this is being dealt with.”
Ms Rayner is one of several MPs who in recent days has criticised the language used by Prime Minister Boris Johnson in Parliament.
Mr Johnson was criticised for using words like “betray” and “surrender” on Wednesday night, the first day MPs had returned to Parliament after it was prorogued.
Labour MP Paula Sherriff called on the prime minister to “moderate” his language, saying many MPs receive death threats and abuse which “often quote” the same words.
Mr Johnson initially described her safety concerns as “humbug”.
He has since refused to apologise for his language, but insisted he “deplores any threats to anybody, particularly female MPs”.