Boris Johnson is facing mounting pressure over groping claims against a Conservative whip, as two senior female MPs demand a “zero tolerance” approach to sexual misconduct in the party.

Chris Pincher quit his role as deputy chief whip after allegedly groping two men – and the PM is being urged to kick him out of the Tory parliamentary group.

Downing Street defended the prime minister’s decision to appoint Mr Pincher to the whip role earlier this year despite concerns reportedly expressed at the time, saying it was “not appropriate to stop an appointment on the basis of unsubstantiated allegations”.

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Tory MPs Karen Bradley and Caroline Nokes – the only two Conservative female chairs of backbench select committees – took aim in a joint letter at what they called the “inconsistent and unclear approach” by the party to such incidents.

In a letter to chief whip Chris Heaton-Harris, they said the current approach risked causing “serious reputational damage” to the party and the government and urged him to introduce a code of conduct for all Conservative MPs “which is clear in terms of the expectations of behaviour”.

They added: “In the meantime we ask that you employ a policy of zero tolerance on these issues and to ensure a thorough investigation is carried out in each and every case.

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“Once an investigation has been completed, a decision should be taken about returning the whip, but in the meantime anyone subject to such an investigation should not be allowed to sit as a Conservative MP and represent the party in any capacity.”

The PM’s spokesman rejected the suggestion that Mr Johnson does not take sexual assault claims seriously, and said he believed the behaviour was “unacceptable”.

“The prime minister does think he’s [Mr Pincher] done the right thing in resigning and has accepted that resignation,” the spokesman said.

In his whip’s role, Mr Pincher had been responsible for party discipline and ensuring the smooth running of government business at Westminster.

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Sky News has been told that the episode had added to the number of Tories who have lost faith in the PM – on top of the 148 who delivered a no-confidence verdict in a bruising revolt last month.

One unnamed Tory MP told Sky News: “Parish was nothing compared to this. No way he can keep the whip.”

That was a reference to Neil Parish – whose resignation as an MP after he admitted watching pornography in the Commons chamber resulted in the Tories’ by-election mauling by the Liberal Democrats in Tiverton and Honiton.

The MP added: “I know three or four people who voted confidence in the PM who have changed their minds.”

Labour has also called for Mr Pincher to have the whip removed – meaning he would be kicked out of the party’s parliamentary group and sit as an independent.

News correspondent

As Boris Johnson returns from his world tour, Chris Pincher’s resignation will add to his domestic woes.

The last thing a prime minister, fighting to save his own leadership, needs is to lose an ally responsible for keeping rebellious backbenchers in check.

The fact Mr Johnson is now looking for a deputy chief whip and a party chairman (as well as an ethics adviser) makes a reshuffle sooner rather than later feel more likely.

The bigger problem though for Mr Johnson is the debate Mr Pincher’s resignation will inevitably reignite around the culture in Westminster, and specifically in the Tory Party.

A week since two by-elections triggered by porn-watching and child sex abuse, the Conservatives are facing fresh questions about why so many Tory MPs are finding themselves embroiled in scandal.

Comparisons with the sleaze of the John Major years continue to be made – a pattern of controversy followed by bruising by-election.

Labour will no doubt be eyeing a potential by-election in Mr Pincher’s seat of Tamworth, a constituency they lost to the Tories in 2010.

There is currently a 19,600 majority in Tamworth, and less of a swing would be needed for it to change hands than in Tiverton and Honiton.

We are not there yet, as Mr Pincher would have to not only lose the whip but stand down as an MP (or be pushed out under the recall procedure) to trigger a by-election.

But if the mood is against him, things could move very quickly.

‘I embarrassed myself’

Mr Pincher resigned as deputy chief whip after admitting he had drunk “far too much” and “embarrassed myself and other people” on a night out.

“I apologise to you and to those concerned,” the Tamworth MP said in a letter to the PM.

Mr Pincher is said to have “groped two men in front of others” at the Carlton Club in the St James’s area of central London, a government source told Sky News. One of them was an MP, and the second may also be a member of parliament, the source added.

Earlier, Wales Secretary Simon Hart told Sky News that he was “very sad” about the situation, but refused to be drawn on Mr Pincher’s future.

He added: “I think it is entirely right, that the chief whip and others take a view today about what is the appropriate course of action.”

Mr Hart said that “if there are those who are victims of this or who wish to raise complaint, they can do so”.

He added: “This is not the first time, I fear it possibly won’t be the last. This happens in workplaces from time to time.”

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