Nikolas Cruz, who murdered 17 students and staff with a semi-automatic rifle at a Florida high school, has been formally sentenced to life in prison.

A jury voted last month to spare Cruz, 24, the death penalty, instead choosing life in prison without possibility of parole for one of the deadliest mass shootings in US history.

Cruz pleaded guilty last year to premeditated murder for his rampage on February 14, 2018, then faced the three-month penalty trial earlier this year.

Broward County Circuit Judge Elizabeth Scherer agreed to a prosecution request to first allow relatives of Cruz’s victims to address the court before the sentence was handed down on Wednesday.

The sentencing proceedings began on Tuesday with victim impact statements.

A number of victims’ relatives castigated the jury’s decision and criticised a state law requirement that all 12 jurors be unanimous in order to sentence a convicted person to be executed.

Some relatives also chided Cruz’s defence lawyers, who fruitlessly objected to the judge on Tuesday, noting that Cruz had a constitutional right to legal representation.

Many victims’ relatives directly addressed Cruz, who sat inscrutable behind large spectacles and a COVID-19 mask at a table alongside his public defenders.

Anne Ramsay, the mother of 17-year-old Helena Ramsay, told him he was “pure evil”; Inez Hixon called him a “domestic terrorist” for killing her father-in-law, school athletics director Chris Hixon.

Cruz was 19 at the time of his attack on Marjory Stoneman Douglas High School in Parkland, about 50 kilometres north of the courthouse in Fort Lauderdale. He had been expelled from the school.

Some of the survivors went on to organise a youth-led movement for tighter gun regulations in the United States, which has the highest rate of private gun ownership in the world and where mass shootings have become recurrent.

– AAP