The Ministry of Defence has launched an investigation into a video which appears to show soldiers firing shots at a picture of Jeremy Corbyn.
The video, which has not been verified by the BBC, appears to show four paratroopers using a portrait of the Labour leader for target practice.
Brigadier Nick Perry said the Army was conducting a full investigation.
“The video shows totally unacceptable behaviour and a serious error of judgment,” he said.
Brig Perry, commander of 16 Air Assault Brigade, said there were currently 400 soldiers from his brigade working with Nato and Afghan partners in Afghanistan, where the footage is thought to have been filmed.
He said they were doing an “outstanding job in theatre” but this incident would be fully investigated.
He stressed the Army was, and always would be, an apolitical organisation.
A Labour Party spokesman called the behaviour alarming and unacceptable but said they have confidence in the Ministry of Defence to investigate and act on this incident.
The prime minister’s official spokesperson said Theresa May was aware of the video but had not watched it, and had called it “clearly unacceptable”.
Conservative MP Tom Tugendhat, a former lieutenant colonel who served in Iraq and Afghanistan, said the video was “disgraceful”.
Rory Stewart, Conservative minister for prisons, told the BBC’s Victoria Derbyshire that it was “completely wrong” and the soldiers’ behaviour was “outrageous”.
“They should not be political – they are there to defend the country and the Queen,” he said.
The emergence of the video comes at a time of heightened alarm about the safety of MPs as tensions rise over Brexit.
Labour MP Jess Phillips tweeted: “This is absolutely hideous and irresponsible under this or any climate.”
And Angela Rayner, Labour’s shadow education secretary, said she hoped the investigation would be conducted “thoroughly and the conclusions made public”.
It is not known when the footage was filmed.
It is believed the short clip first circulated on Snapchat before being posted on Twitter.