Boris Johnson today ordered the Cabinet Office to investigate allegations an ex-minister was sacked over 'Muslimness'.
The PM has instructed the civil service to carry out a probe after the extraordinary claims from Tory MP Nusrat Ghani.
The move came after Nadhim Zahawi and Sajid Javid joined calls for a 'proper' inquiry - with others suggesting it should be fully independent.
A No10 spokeswoman said this morning that Mr Johnson took the allegations 'very seriously'.
'The Prime Minister has asked the Cabinet Office to conduct an inquiry into the allegations made by Nusrat Ghani MP,' the spokeswoman said.
'At the time these allegations were first made, the Prime Minister recommended to her that she make a formal complain to CCHQ. She did not take up this offer.
'The Prime Minister has now asked officials to establish the facts about what happened.
'As he said at the time, the Prime Minister takes these claims very seriously.'
A thinktank has called on the Government to bring in the Equality and Human Rights Commission (EHRC) to investigate the claims.
The Runnymede Trust, a race equality thinktank, says the issue is too important to be left to a civil servant-led inquiry.
Ms Ghani alleged that chief whip Mark Spencer said her faith was partly responsible for her getting the boot in 2020.
No10 claimed she had made the PM aware of the allegation at the time but declined an invitation to ask for a full investigation. But Ms Ghani contradicted their version of events, saying the PM refused to get involved and tried to fob her off.
Dr Halima Begum, chief executive of the Runnymede Trust, told the Guardian: 'This is an incredibly serious situation. At a bare minimum, the allegation that a minister of the crown was fired for her so-called 'Muslimness' would represent a flagrant challenge to our equalities and labour laws.
'The facts and questions about the legality of what has happened here must be urgently investigated by the very highest authority. This cannot simply be left for another civil service inquiry. If the allegations are proven to be true, Nusrat would have been subjected to grossly discriminatory behaviour.
'[Her] distress will be felt by every one of the 3 million Muslims in the country, as well as every member of our religious minority communities. All of the political parties need to do more to demonstrate zero tolerance for discrimination, and to prove that religious minorities in this country are respected regardless of their faith.'
Caroline Nokes, chair of the Commons women and equalities committee, described the treatment of Ms Ghani as 'appalling'.
She also backed calls for the EHRC to launch an investigation and told the Telegraph: 'Her faith has never made me (or any other colleague) 'uncomfortable'.
'At the very least EHRC should have a look at this.'
An EHRC spokesman stated the Commission is still examining the Conservative Party's handling of the Singh inquiry into Islamophobia complaints last year and suggested a full investigation may take place.
The spokesman added: 'If we are not satisfied with progress we will review our decision [not to begin an immediate review] and do not rule out the use of our legal powers.'
Dr Halima Begum, director of the Runnymede Trust, said the incident 'must be urgently investigated by the very highest authority'. She added: 'This cannot simply be left for another civil service inquiry'
