The army and police are investigating after a soldier sent a death threat to the shadow education secretary, Angela Rayner, defence officials have said.

The commander of the British field army, Lt Gen Ivan Jones, and the defence secretary, Ben Wallace, denounced the message, which was posted on Twitter on Wednesday, as MPs pleaded with the prime minister to tone down his Brexit rhetoric, saying they feared it would incite violence against them.

Posting an image of the message online on Thursday, Rayner said it was just an example of the “usual vile tweet I get daily”. It read that she will “perish when civil war comes”, suggesting that people who voted for Brexit would be “gunning for blood if we don’t leave”.

Jones apologised and confirmed his belief that the tweet was posted by a serving soldier.

Lt Gen Ivan Jones - Commander Field Army (@field_army) I would like to apologise personally to @AngelaRayner MP and anyone affected by appalling tweets from @GoshawkDaniel, a British soldier. He does not represent the remarkable men and women in @BritishArmy who serve this nation. Rest assured this is being dealt with. @BWallaceMP pic.twitter.com/R4UHz6kU1T

Wallace called the tweet unacceptable and confirmed that police had been called in.

Rt. Hon Ben Wallace MP (@BWallaceMP) This is an unacceptable tweet from a member of the @BritishArmy to @AngelaRayner. This foul language goes against the values of the Armed Forces and is now being investigated by the Army and civilian police https://t.co/LYe4d2PkoF

On Friday, an army spokeswoman said: “We are aware of a post made on Twitter in reply to an MP. This incident has been referred to the civilian authorities to investigate. The Army is a politically neutral organisation and holds its people to the highest standards.”

The prime minister has come under pressure to tone down his language when debating Brexit, having recently made routine use of rhetoric more commonly associated with war; referring to “betrayal” and terming the Benn Act, which requires him to seek an extension in the absence of a deal, the “surrender bill”.

His senior aide, Dominic Cummings, has also said that only way the issue of threats and abuse will be solved is if MPs “respect” the result of the EU referendum.

On Friday, the former home secretary Amber Rudd said Johnson’s language, which some have suggested is part of a calculated electoral strategy, “does incite violence”. She said: “It’s the sort of language people think legitimises a more aggressive approach and sometimes violence.”