The chair of the undercover policing inquiry has revealed he has been diagnosed with motor neurone disease.
Sir Christopher Pitchford said his diagnosis last November meant he would eventually have to stand down from the complex and already-delayed probe.
In a personal statement, the senior judge said he regretted he would not be able to complete the inquiry's work.
The inquiry said that a successor would be identified to begin the work of taking over in due course.
The inquiry headed by the Court of Appeal judge is investigating serious allegations relating to undercover operations - some dating back 40 years.
The inquiry said both the home secretary and lord chief justice had been informed of Sir Christopher's illness.
It is said not to have affected his running of the inquiry so far - although the judge's physical symptoms have been becoming apparent.
The home secretary has been asked to appoint another judge to work alongside Sir Christopher and succeed him when he ultimately stands down.
In a personal statement, Sir Christopher said: "I very much regret that my diagnosis and the progression of my physical symptoms mean that I shall not be able to complete the work of the inquiry.
"However, I wish to assure the inquiry's core participants and the public that the inquiry's work continues unabated and that, with the support of the home secretary and the lord chief justice, for which I am grateful, the transitional arrangements that are being put in place will ensure its continuity when the time comes for me to step down as chairman."
Theresa May appointed Sir Christopher in 2015 when she was home secretary, asking him to report by the summer of next year.
The inquiry is already substantially behind schedule because of a complex legal stand-off with Scotland Yard over how many former officers from the controversial unit at the heart of some of the allegations should give evidence.
That issue is expected to come to a head in a public hearing later in the spring which is expected to still take place with Sir Christopher at the helm.