The Prime Minister has turned down requests from survivors of the Grenfell Tower fire to appoint a diverse, decision-making panel to sit alongside the head of the public inquiry.
Bereaved family members delivered a petition, signed by more than 22,000 people, to Downing Street earlier this month, calling for an independent panel amid doubts over Sir Martin Moore-Bick's suitability.
It is feared that the retired Court of Appeal judge lacks first-hand experience of the complex cultural factors underpinning the tragedy.
Only Theresa May can authorise a change in the inquiry's format under the Inquiries Act 2005, and in a letter on Wednesday Sir Martin said it would be wrong for him to advise her either way.
Mrs May said on Friday that additional panel members should not be appointed in order for the first stage of the inquiry to be completed "as quickly as reasonably possible".
In a letter to Sir Martin, she said she felt the probe had "the necessary expertise to undertake its work".
She continued: "I am also very conscious of the need for the Inquiry to complete its initial report as quickly as reasonably possible. I therefore consider that additional panel members should not be appointed at this stage."