Mark Field has been suspended as a Foreign Office minister after a video showed him pushing a female Greenpeace activist against a pillar and grabbing her neck while she protested at the chancellor’s Mansion House speech.

Police are investigating third-party reports of assault made against Field, who has since apologised to the protester. The MP for the Cities of London and Westminster said he had felt threatened when the protester walked past him and was worried she might have been armed.

Downing Street said Theresa May had viewed the footage of the incident on Thursday night and decided to suspend him.

The activist, Janet Barker, said on Friday that she was incredulous at his reaction and welcomed the suspension but would not press criminal charges. “I think it is something best dealt with in the court of opinion,” she said.

Barker said Field had pushed her so hard as they reached the door that she had almost fallen. She said he should take anger management classes. “I want him to think about what he did, why he did it and address his behaviour.”

She said she had made no sudden movements or behaved in any way that could have been construed as physically threatening. “I had a phone and a tiny handbag, which was open and full of leaflets,” she said. “The only thing I was armed with was peer-reviewed science.”

Labour challenged the Conservatives to suspend Field from the party and the whip pending the outcome of the investigation into his behaviour.

The prime minister’s spokeswoman said: “She found it very concerning. The police have said they are looking into reports over this matter.

“He will be suspended as a minister while this investigation takes place. Mansion House are looking into the breach of security that took place and we believe it is right they are now reviewing their security arrangements.”

May is understood to have spoken to her chief whip, Julian Smith, after viewing the footage, and Smith told Field he had been suspended mid-morning on Friday.

Asked whether Conservative MPs including Peter Bottomley and Johnny Mercer should have been defending Field in the media while he was being investigated, the spokeswoman repeated that the prime minister had found the footage to be a cause for concern.

Field was a minister in Jeremy Hunt’s team at the Foreign Office and was supporting him in the Tory leadership contest. Hunt told the BBC: “Mark has issued a full and unreserved apology. He recognises that what happened was an over-reaction.”

Philip Hammond was preparing to deliver his set-piece address in the City of London when dozens of Greenpeace activists interrupted him to give an alternative speech about the climate crisis, video footage shows.

In a statement released in the early hours of Friday before his suspension, Field said he had reacted “instinctively”. He said he “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 I needed to act decisively to close down the threat to the safety of those present.”

Field said he had referred himself to the Cabinet Office to “examine if there has been a breach of the ministerial code” and that he would cooperate fully with its investigation.

Guests at the event raised concerns about security because the protesters appeared to have evaded airport-style checks.

“It seemed to be very difficult to get the security at the venue to do their job once it became clear that something was happening,” one person who was present complained.

Attendees said they feared a terrorist attack was taking place in the seconds after the activists burst in, but quickly realised they were climate protesters.

The incident involving Field is understood to have taken place on the opposite side of the hall from where the majority of protesters were stopped trying to give Hammond and the Bank of England governor, Mark Carney, their alternative speech and distribute leaflets.

One guest said Field’s actions had “raised eyebrows” and left those who saw the incident shocked.

Asked what happened when the protesters entered the hall, a financier said: “People stopped talking and listened politely. A lady kept reading the same sentences over and over again, but she was very polite. We listened for three to four minutes, said thank you very much and clapped. Then they were asked to leave and security took them out.

“They came straight in. I don’t know how they managed to do it. I brought my passport with me for an ID check when I arrived but it did not happen, but there were security scanners,” he said.

Some of the male protesters – who were wearing black tie – appear to have sat down at the dining tables, before getting up as a group and leaving to join the female protesters who were wearing distinctive red dresses with a sash saying “Climate emergency.”

Questions were being asked in political circles about whether the chancellor would return to Mansion House given the lack of security. “You’ve had protests when George Osborne has appeared and now this. Liz Truss was there, but if she becomes chancellor it’s not a certainty she’ll be back to give a speech this time next year,” a guest said.

The City of London Corporation, which organises the dinner, said it was investigating the security breach.

Greenpeace did not provide details of how the activists gained entry to Mansion House.

Field’s situation is complicated by the fact that he has been responsible for the UK government’s response to the crisis in Hong Kong, where he called this week for allegations of “inappropriate use of force by the Hong Kong police” to be properly investigated. Questions about his own conduct could make it harder for him to press for restraint in Hong Kong.