Mrs May will represent the Government alongside Northern Ireland secretary Karen Bradley at St Anne’s Cathedral this afternoon. Labour leader Jeremy Corbyn will also be present at the funeral.
In Mrs May’s absence, David Lidington, the Prime Minister’s de facto deputy, will answer questions in the House of Commons. He has previously stood in for Mrs May, while Emily Thornberry will take PMQs for Labour.
Ms McKee was killed during clashes between police and New IRA dissidents on the Creggan estate in Londonderry on April 18.
Speaking ahead of her funeral, her mother Joan, brothers Gary and David, and sisters Joan Hunter, Nichola Corner and Mary Crossan paid tribute to her as “a best friend and confidante”.
In a statement, the family said: “On Thursday 18th April our beautiful Lyra was taken from us.
“A daughter, a sister, an aunt, a great-aunt, a partner, a niece, a cousin, and above all, a best friend and confidante to so many of us.
“A friend to all, a gentle innocent soul who wouldn’t wish ill on anyone. Such a warm and innocent heart, she was the greatest listener, someone who had time for everyone.”
The service will include tributes from friends and a reflection by Fr Martin Magill, Catholic parish priest at St John’s parish on the Falls Road in West Belfast.
The McKee family added: “She was a smart, strong-minded woman who believed passionately in inclusivity, justice and truth.
“Lyra spoke to and made friends with anybody and everybody, no matter what their background, those of all political views and those with none. This openness, and her desire to bring people together, made her totally apolitical.”
They said: “We would ask that Lyra’s life and her personal philosophy are used as an example to us all as we face this tragedy together.
“Lyra’s answer would have been simple, the only way to overcome hatred and intolerance is with love, understanding and kindness.”
Sara Canning, Ms McKee’s partner, said previously: “Our hopes and dreams and all of her amazing potential was snuffed out by this single barbaric act.”
Ms McKee’s funeral is intended to be a cross-community, cross-border and multi-cultural service, while members of the National Union of Journalists (NUJ) will form a guard of honour.
Various political and community leaders are due to attend, including Taoiseach Leo Varadkar, President of Ireland Michael D Higgins and Irish Minister for Foreign Affairs Simon Coveney.
Those attending were asked to wear Harry Potter and Marvel Comic merchandise in tribute to the journalist’s love of both works.
The New IRA admitted responsibility for the murder on Tuesday in a statement given to The Irish News.
Using a recognised code word, the group offered “full and sincere apologies” to Ms McKee’s family and friends, claiming: “We have instructed our volunteers to take the utmost care in future when engaging the enemy, and put in place measures to help ensure this.”
Sinn Fein leader Mary Lou McDonald condemned the New IRA “unreservedly” following the statement.
She told BBC Radio 4’s Today programme: “The war in Ireland is over, the conflict is over, the IRA arms have been destroyed or put beyond use.
“We have a new democratic dispensation and it is so important we make that work, that institutions of government work.”
She added: “They are a tiny, unrepresentative group that have no plan, that have no strategy.
“If their alibi (for the killing of journalist Lyra McKee) is they are seeking to advance the cause of republicanism, they are very far off the mark.”
Northern Ireland secretary Karen Bradley rejected the New IRA’s apology in a statement to the House of Commons yesterday relating to the death of the Miss McKee.
She said: “It remains the case across Northern Ireland that small numbers of dissident republican terrorists remain intent on killing.
“What we have seen in the days since Lyra McKee’s death is that the communities that they claim to represent and seek to control don’t want them.”
Ms Bradley later said: “To those responsible for this act of terrorism, we say we have heard your excuses and your hollow apologies. No-one buys it.
“This was no accident. There is nothing that can justify this murderous act and you are being called out for what you really are.”
The New IRA is an amalgam of armed groups opposed to the peace process and it recently claimed responsibility for parcel bombs sent to London and Glasgow in March.
Police believe the violence was orchestrated in response to an earlier search by officers aimed at averting imminent trouble associated with the week’s anniversary of the Easter Rising.