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A federal court has found Ohio's three-drug lethal injection process unconstitutional, putting three impending executions on hold.

Magistrate Judge Michael Merz ruled against Ohio's use of a sedative that has caused problems in executions in other states such as Arizona.

The state was also banned from using drugs that paralyse inmates and stop their hearts.

Next month's execution of Ronald Phillips was subsequently delayed.

The judge sided with Phillips and two other death row inmates that the sedative midazolam could not pass a US Supreme Court constitutional bar of causing "substantial risk of serious harm".

His ruling also prohibited the state from using the second and third drugs in the method: rocuronium bromide and potassium chloride.

Judge Merz said using those drugs was "completely inconsistent with the position" the state took when it said it would no longer use them in executions.

The state contended the three-drug process was constitutional, citing a Supreme Court ruling last year.

Phillips would have been the first inmate to be executed in the Midwestern state for three years.

In January 2014, inmate Dennis McGuire gasped and snorted during the 26 minutes it took for him to die.

It was the longest execution since Ohio resumed executions in 1999.

Ohio then ended the two-drug method used on McGuire and struggled to find new supplies after drug companies began banning the use of their drugs for executions.

In October, Ohio said it would use midazolam with rocuronium bromide, which causes paralysis, and potassium chloride, which stops the heart.