It is widely expected that Home Secretary Theresa May will make an announcement on an independent judge-led inquiry into the murder of Daniel Morgan.
The private investigator was found with an axe in his head in a London pub car park in 1987.
It has been claimed that police corruption prevented the murder from being solved despite six criminal investigations.
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The talks descended into crisis on Friday when Canada's trade minister Chrystia Freeland walked out of meetings.
Police in New York are still investigating what the substance is and whether it could be harmful.
The Sherlock actor made the comment while he received an award for his humanitarian work.