Theresa May today said it is 'unacceptable' for the Commons Speaker to call a senior Cabinet minister a 'stupid woman' and 'f*****g useless', if the allegations are true.

John Bercow is said to have muttered the foul-mouthed phrase about Andrea Leadsom during an angry exchange in the Commons after PMQs on Wednesday.

Mr Bercow is already facing multiple allegations of bullying his staff,, and the latest allegation has renewed calls for him to quit from the influential parliamentary role.

Commenting on the allegation today, the Prime Minister's official spokeswoman said: 'We have seen the alleged remarks, and clearly the Prime Minister thinks they are unacceptable, and if an official complaint is made it should be properly investigated.'

Friends of Mrs Leadsom told MailOnline she is not currently planning to make a complaint.

The Parliamentary Commissioner for Standards, the body which has the power to investigate such an allegation, told Mail Online it has not received a formal complaint yet.

Labour MP Barry Sheerman rallied to Mr Bercow's defence, accusing Tory whips of spinning the row and calls on Mrs Leadsom to resign.

But his remarks sparked a furious slap down by Tory Scottish leader Ruth Davidson who accused him of 'mansplaining' and sexism.

Theresa May (pictured left, in Wales today) has said the alleged remarks by John Bercow (pictured, right, in the House of Commons) are unacceptable and called for an investigation into them

A witness claims he saw Mr Bercow describe Mrs Leadsom as a ‘stupid woman’ under his breath, before adding that she was ‘f***ing useless’.

What are the claims of bullying and bad behaviour made against John Bercow? John Bercow has been hit by a number of bullying claims, all of which he denies. Here are the allegations: Andrew Sinclair, the former Speaker's Secretary: Said he left his post in 2010 after he was subjected to angry outbursts, foul- mouthed tirades and mimicry by John Bercow. He said was 'gagged' and stopped from speaking out about his time in the role as part of an £86,000 early retirement payoff. David Leakey, former Black Rod: Mr Leakey, who retired last year, said staff were ‘terrified’ of the Speaker. Kate Emms, John Bercow's former private secretary: She took a job as Mr Bercow's private secretary in May 2010 but left her role less than a year later in February 2011 amid claims she was bullied by the Speaker. Her colleagues alleged that this was because of the behaviour of Mr Bercow after she told staff that she suffered from post-traumatic stress disorder. Colleagues told Newsnight that she was undermined by Mr Bercow in a range of ways – and he seemed to shout at her a lot. Advertisement

A source close to Mrs Leadsom, who was made aware of the alleged remarks by colleagues, said she was ‘stunned’ by the attack, but intended to ‘rise above it’.

A spokesman for the Speaker did not deny the allegation last night, but said it had been an ‘unusual and controversial’ day on which ‘some strong and differing views were expressed on all sides’.

Mr Bercow strongly denies all of the bullying claims made against him.

As the Commons Speaker, Mr Bercow oversees debates in the Commons and is charged with keeping order. It is a very senior role and requires him to be strictly impartial.

The row comes as the Speaker is battling to hang on to his job amid claims he has bullied several members of staff.

And Tory MP Andrew Bridgen, who reported him to the parliamentary standards watchdog over bullying allegations, called for Mr Bercow to quit.

He said: ‘John Bercow claims to support feminism and diversity, and virtue signals at every opportunity, but obviously it’s not the case when it comes to Tory MPs.

‘I think it’s fair to say that his behaviour is becoming more strange by the day. John Bercow should go now before he damages our parliamentary democracy any further.'

And the allegation sparked a Twitter row between Ms Davidson and Mr Sheerman.

Mr Sheerman tweeted: 'Outrageous treatment of the House of Commons by Leader of the House Andrea Leadsom spun by Tory Whips as fault of Speaker! Leadsom should now resign!'

But Ms Davidson hit back on Twitter writing: 'Is BarrySheerman REALLY trying to mansplain why a professional woman, when sworn at and called useless at her place of work, should take the blame?

'Women, know your place!'

According to colleagues, Andrea Leadsom (pictured on her way to cabinet earlier this year) was left 'stunned' when she was told about the comments allegedly directed at her by John Bercow

Mr Sheerman, 78, replied by saying Mrs Leadsom is no 'shrinking violet' but this sparked another furious response from Ms Davidson who wrote: 'Thank you for explaining to me who my colleague, Andrea Leadsom, is.

'How would my silly little female head have known that without your help? I still think the idea that you think SHE should lose HER job for being publicly sworn at and upbraided is ridiculous!'

She added: 'Yeah, your right. "Stupid woman" and "f***ing useless" is just out-and-out abusive.

'Funny there's been no denial, even though you're calling for HER head....'

A string of Mr Bercow's former employees have come forward to claim they were bullied by him.

Scottish Tory leader Ruth Davidson got in a Twitter row with Labour MP Barry Sheerman over the John Bercow allegations

The Labour MP hit back at claims he is sexist during the lengthy Twitter exchange

Ruth Davidson accused the Labour MP of trying to mansplain to her during the tense stand off

Labour Mp Barry Sheerman is a strong supporter of John Becow as Speaker

Ruth Davidson - who is tipped as a future British Tory leader - hit back at the English MP with sarcasm during the back and forth

They claim the Speaker, who oversees debates in the Commons and is tasked with maintaining order, would lose his temper and subject them to mockery and fiery tirades.

Angus Sinclair, who left his post as Speaker's Secretary in 2010 after receiving a payment of £86,250, said he was the victim of angry outbursts, foul- mouthed tirades and mimicry.

Former Black Rod David Leakey, who retired last year, has said staff were 'terrified' of the Speaker.

Mr Bercow has faced calls to step down in the wake of the allegations while many MPs want the accusations to be fully investigated.

But MPs earlier this week voted to block an inquiry by the into Mr Bercow’s conduct - sparking fury among his critics.

The Parliamentary Commissioner for Standards, Kathryn Stone, had asked for permission to investigate allegations of bullying against the Commons Speaker.

But the Commons committee on standards voted to reject her plea, because the claims against him are more than seven years old

What role does the Speaker of the Commons play? The House of Commons Speaker oversees debates and imposes order on MPs. The Speaker is the chief officer and highest authority of the House of Commons and must remain politically impartial at all times. They can use their role to fundamentally shape what and how MPs debate by deciding whether or not to grant urgent questions on topics - forcing ministers to come to the commons and explain their policies to MPs. They are elected by fellow MPs and must remain strictly impartial in chairing debates. The Speaker also represents the Commons to the Monarch, the Lords and other authorities and chairs the House of Commons Commission. John Bercow, the Tory MP for Buckingham, has been Speaker since 2009. To get rid of a Speaker MPs must table a motion of no confidence in them, which is then debated and voted on by MPs. Advertisement

The controversial decision has raised fresh questions about the system for regulating conduct at Parliament.

Seven 'lay' members serving on the powerful committee were barred from voting on the issue, and are said to have been concerned at the outcome.

And it renewed calls among some MPs for Mr Bercow to do the 'honourable thing' and refer himself to the Standards Committee for investigation.

An inquiry led by a former judge is looking at wider cultural issues at Westminster, but will not consider specific cases.

Mr Bercow, who has been Commons Speaker since 2009, originally promised to serve just nine years, which would mean he shuld step down on June 22.

But challenged baout hwen he planned to go in a tense showdown in the Commons last week, he signalled his intention to serve anther four years - until the next General Election.

MPs close to Mr Bercow believe he could opt to quit rather than cling on to the crucial role until the next election in 2022.

However, he is said to want to go 'on his own terms' after serving a decade in the chair.

Mr Bercow's spokeswoman said: 'Wednesday was an unusual and controversial day in how business was handled in the House by the government - and some strong and differing views were expressed on all sides on the subject.

'The Speaker treats his colleagues with respect and strives at every turn to facilitate the House of Commons.'

In a statement Andrea Leadsom said: ‘I am focused on ensuring that anyone who is bullied or treated unfairly in Parliament is able to come forward and have their concerns and complaints dealt with in an independent, rigorous and fair manner.’