Downey Jr is shown in a police photo after arrest

Producers have admitted it will be a "scramble" to complete this season's filming without Downey Jr, but pointed out that some scenes were already recorded.

"He was contracted for every episode this season and he might do that if we use the scenes we already have in the can," said Chris Alexander, of 20th Century Fox.

Mr Alexander added that Downey Jr's return to the show next series "was never negotiated or discussed".

A reported marriage proposal from Downey Jr's character to Ally McBeal was never planned, he said.

Setbacks

Police gave no details on the circumstances of Downey Jr's arrest and would not reveal the type of drug he is accused of using.

Downey won a Screen Actors Guild award for best actor in January

But a California Department of Corrections spokesman said the actor would be treated like any other person on parole.

He will enter a live-in drug treatment facility for at least six months for breaking parole.

This is the latest in a string of setbacks for the actor.

In 1996 he was stopped for speeding and police found cocaine, heroin and a pistol in the vehicle.

A month later he was found unconscious and underwent treatment at a drugs centre.

In August 1999 he was sentenced to three years in prison for violating his probation by missing scheduled drugs tests.

Downey with Ally McBeal star Calista Flockhart

He spent a year in prison until August 2000 but was arrested on 25 November at a hotel in Palm Springs, Florida, after police received an anonymous tip.

He is due to appear in court on Monday in that case.

David E Kelley and other producers on Ally McBeal had been supportive of Downey Jr until his latest arrest.

A spokesman for Downey Jr said the actor was now "in charge of his own destiny".