Foreign Office Minister Mark Field has come under fire after video footage showed him physically removing a climate change protester from a dinner in the City of London.
The video clip shows Mr Field stopping a female activist by pushing her against a column at Mansion House before holding her by the back of the neck and forcefully walking her out of the room.
It came as protesters interrupted a black-tie event where Chancellor Philip Hammond was giving a speech.
Mr Field has referred himself to the Cabinet Office for investigation and has apologised “unreservedly” to the activist.
Conservative Party chairman Brandon Lewis said the party will investigate the incident.
In a statement to ITV, Mr Field is quoted as saying: “In the confusion many guests understandably felt threatened and when one protester rushed past me towards the top table I instinctively reacted.
“There was no security present and I was for a split-second genuinely worried she might have been armed.
“As a result I grasped the intruder firmly in order to remove her from the room as swiftly as possible.”
He added that he “deeply” regretted the incident and would co-operate fully with a Cabinet Office investigation.
Speaking on Good Morning Britain, Mr Lewis said: “I’ve spoken to the chief whip this morning and there will be an investigation from our end.”
He said it was “very hard to defend” the footage, adding: “It’s hard for anybody to look at that and not be astonished at what they have seen.”
But he said the investigation would look into the “full details of what happened”.
After the footage emerged, Tory MP Sir Peter Bottomley told the Press Association that Mr Field had done nothing wrong.
He said: “The woman clearly was trying to create a fuss.
“Most viewers would say it’s good that she didn’t succeed.”
Asked if Mr Field had been heavy-handed with the protester by grabbing her by the back of the neck, Sir Peter replied: “No, he reversed her direction and she looked as though she went willingly.
“I think there’s no reason to criticise Mark Field … Of course it wasn’t an assault, it was a reversal of direction.”
But there was condemnation from other MPs who saw the clip of Mr Field.
Labour MP Jess Phillips tweeted: “She posed no credible threat from what I can see. There is very little else that could justify this and anyone can see that this could have been done without physical contact. Every MP has to deal with protest and conflict, it is done with words. To watch this is so, so awful.”
Fellow Labour MP Dawn Butler said it was “horrific” and called for Mr Field to be sacked or suspended.
Ms Butler, who is shadow women and equalities secretary, tweeted: “Conservative Foreign Office Minister Mark Field violently grabs a woman as she protests about climate change at the bankers’ banquet. This appears to be assault. He must be immediately suspended or sacked. Due to Violence against women.
“I’m sure I’m not the only one who is wondering why no one intervened. So much violence does not seem justified. An investigation needs to take place as soon as possible.”
Liberal Democrat MP Chuka Umunna described Mr Field’s actions as “totally unacceptable”, while Independent MP Sarah Wollaston said it was “absolutely shameful, a male MP marching a woman out of a room by her neck”.
Tory MP George Freeman tweeted: “This looks appallingly rough handling of a woman climate protester in a dress. But before everyone rushes to instant armchair judgement can I suggest that all of us who weren’t there & don’t know what was said or done just wait a few hours to hear what those who *were* there say.”
Conservative former Middle East minister Alistair Burt was shown the clip on BBC Two’s Newsnight.
He said: “I’ve no appreciation of the context … Mark will answer for himself but it looks to be a very difficult situation for everyone concerned.”
When contacted by the Press Association, Foreign Secretary Jeremy Hunt, who is Mr Field’s boss, responded with a text saying “sorry can’t talk now”.
City of London Police said they were looking into a “small number” of third-party reports of an assault at the event.
Mr Hammond had barely started his address when activists interrupted on Thursday night.
The disruption lasted several minutes before a slow hand-clap broke out among the seated guests, and the Chancellor got back to his feet at 9.05pm.
To a round of applause, Mr Hammond then said: “The irony, of course, is that this is the Government that has just led the world by committing to a zero-carbon economy by 2050.”
Greenpeace later said 40 of its activists, some of whom wore red evening dresses with sashes that read “Climate emergency”, had interrupted the event, which was being broadcast live on television.
Footage later released by the environmental group shows a line-up of protesters, including men wearing black suits and bow ties and women in red dresses and sashes, walking alongside the building, then rushing up a set of stairs and streaming into the dining hall.
A spokesman said he would not comment on how the group managed to evade security to get into the high-profile event.
In response to the video of Mr Field, the organisation tweeted: “Instead of assaulting peaceful women protesters, @MarkFieldUK would be better off spending his time tackling the #ClimateEmergency.”
Greenpeace said the activists had wanted to deliver a speech on how the current system has failed.
Mr Hammond paused his address at the request of an official in the dining hall who asked him to let security and other staff clear the noisy activists away.
The sound of alarms could be heard in the background.
Senior captains of industry and top City executives were among the invited guests.
Retiring Bank of England Governor Mark Carney later made his final Mansion House speech, which was about the future of finance.
He spoke about a new economy driven by changes in technology, demographics and the environment.
A City of London Police spokesman said: “We have received a small number of third party reports of an assault taking place at the event. These reports are being looked into by police.”