Mark Field is under pressure to quit as a Foreign Office minister after he was seen physically removing a climate change protester from a dinner in the City of London.
Conservative Party chairman Brandon Lewis said he had spoken to Chief Whip Julian Smith and there would be an investigation.
City of London Police said they were looking into a “small number” of third-party reports of an assault at the Mansion House event.
Labour branded video footage of the incident “horrific” and called for Mr Field to be immediately suspended or sacked from his job in leadership hopeful Jeremy Hunt’s ministerial team.
Greenpeace protesters disrupted the black-tie event, where Chancellor Philip Hammond was speaking, to demand action to tackle the climate emergency.
Mr Field has referred himself to the Cabinet Office for investigation and has apologised “unreservedly” to the activist.
Tory Party chairman Mr Lewis told ITV’s Good Morning Britain: “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.”
In a statement to ITV, Cities of London and Westminster MP Mr Field said: “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.
Tory MP Sir Peter Bottomley defended Mr Field’s actions, telling the Press Association he had done nothing wrong.
Sir Peter said: “The woman clearly was trying to create a fuss.
“Most viewers would say it’s good that she didn’t succeed.”
Sir Peter, whose wife Virginia – now Baroness Bottomley – was assaulted while health secretary, said if a police officer had done what Mr Field did “would there have been a fuss? The answer is no.”.
But Labour’s shadow women and equalities secretary Dawn Butler described the incident as “horrific”, adding “he must be immediately suspended or sacked”.
She added: “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.”
When contacted by the Press Association, Foreign Secretary Mr Hunt, who is Mr Field’s boss, responded with a text saying “sorry can’t talk now”.
Greenpeace 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, walking alongside the building, then rushing up a set of stairs and streaming into the dining hall.
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.
Senior captains of industry and top City executives were among the invited guests to hear speeches by Mr Hammond and Bank of England governor Mark Carney.
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.”
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. My remarks at the Westminster Hall Debate on Women Human Rights Defenders. https://t.co/fmQMtXSTgV
— Mark Field MP (@MarkFieldUK) May 9, 2019
In his ministerial role, Mr Field has spoken up for the rights of protesters and condemned violence against women.