A man was told he would spend the rest of his life behind bars by a British judge on Tuesday for murdering three men in a stabbing spree at a park in an attack last year deemed a terrorist incident.

Libyan Khairi Saadallah, 26, pleaded guilty to murdering the men and stabbing three others with a five-inch knife on the evening of June 20 in the southern English town of Reading.

Eyewitnesses heard him say "Allahu Akbar" ("God is great") during the incident, the Crown Prosecution Service (CPS) said in a statement.

"Khairi Saadallah carried out a brutal, pre-planned attack on groups of friends enjoying an evening in the park where they would have felt completely safe," said Jenny Hopkins, head of the CPS's Special Crime and Counter Terrorism Division.