Campaigners are launching a High Court challenge over the number of unaccompanied child refugees accepted into the UK under the Dubs scheme.
The charity Help Refugees claims the consultation process by which Home Secretary Amber Rudd calculated only 480 should be accepted was "fundamentally flawed".
Actress and activist Juliet Stevenson will be among politicians and other supporters at a "Choose Love" demonstration outside the Royal Courts of Justice in London as the challenge gets under way.
Campaigners, who have called for 3,000 children to be helped, are seeing the application for judicial review as a last chance to get the scheme reopened.
Labour peer Lord Dubs, the architect of the Dubs Amendment, which requires the Government to relocate unaccompanied refugee children from other countries in Europe, is expected to make a statement.
Lord Dubs came to the UK as a refugee from the Nazis and has urged ministers to reconsult local authorities, arguing there were many which had "expressed a willingness to take more child refugees".
Home Office minister Baroness Williams of Trafford told the House of Lords in April when answering a question on numbers that there were 4,000 unaccompanied children already in local authority care with some councils hosting a "disproportionate" number.
Lady Williams added: "We are always glad to hear local authorities coming forward to take children... but it is not as if we have not consulted properly."
Award-winning actress Stevenson, who starred in Bend It Like Beckham and Truly Madly Deeply, is Help Refugees' founder and CEO and has experience of the Calais refugee camps.
Rosa Curling, of law firm Leigh Day, is representing the charity and has criticised the Government, saying there was "no real consultation" with many local authorities, rendering the "woefully low" number of children accepted under the scheme unlawful.
At a preliminary hearing of the case in February, High Court judge Mr Justice Holman said: "There is a huge political dimension to this.
"It is extremely important to establish as soon as reasonably practical whether the number specified is or is not lawful."