Video footage shows Mr Field, who represents the City constituency, pushing the woman against a pillar and then marching her out of the event with his hands on her neck.
Greenpeace has accused the MP of assault and Labour’s shadow minister for women and equalities, Dawn Butler, has called for him to be suspended or sacked.
In a statement Mr Field described the protest as a “major security breach” but apologised to the activist and said he had referred himself to the Cabinet Office for investigation.
“In the confusion many guests understandably felt threatened and when one protester rushed past me towards the top table I instinctively reacted,” he said.
“There was a no security present and I was for a split-second 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.
“I deeply regret this episode and unreservedly apologise to the lady concerned for grabbing her but in the current climate I felt the need to act decisively to close down the threat to the safety of those present.”
Hannah Martin, who was taking part in the Greenpeace demonstration involving around 40 people, told Radio 4 they were peacefully protesting and that the activist intended to hand out leaflets.
“What he did as anybody can see was completely disproportionate and unacceptable, particularly for a sitting member of parliament,” Ms Martin said.
“She was carrying no weapon. There was nobody else in the room who felt it necessary to respond in that way. We clearly stated who we were as soon as we went into the room.
“It was clearly a peaceful protest, it was female led in order to potentially create a calm atmosphere. A member of parliament felt it fit to assault her.”
Defending the actions of his fellow Conservative MP, Sir Peter Bottomley said that Mr Field had sought to “defuse what could have been a tragic situation”.
He said in the context of previous attacks on MPs that “most sensible common sense people would think that’s the right thing to do.”
Shadow cabinet minister Ms Butler, however, said that Mr Field’s actions were “horrific” and added: “This appears to be assault. He must be immediately suspended or sacked. Due to Violence against women.”
Ms Martin replied: “I am quite shocked that Sir Peter Bottomley thinks it acceptable for a man to violently shove a woman up against a marble pillar and then grab her by the neck while push her out.
Former Conservative MP Sarah Wollaston, now an independent, described the incident as”absolutely shameful” and new Liberal Democrat MP Chuka Umunna said it was “totally unacceptable”.
Social media users also highlighted a tweet by Mr Field last month declaring that “the UK remains committed to helping women all over the world to feel safe and protected in the work they do, so they can speak freely and be part of the change we all want.”
The City of London Police said they were called to the scene but had made no arrests, adding that ”the protesters were co-operative and left the premises.”
A spokesman said they had received “a small number of reports of an assault” at the event.
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