Actress Amanda Redman claimed that the series had become "more bland", while Dennis Waterman accused some of the writers of "repeating themselves".
The eighth series of New Tricks secured the show's highest ever ratings, but Waterman was critical of the recent output.
"You have to remind yourself that people aren't as stupid as writers think," he told the Radio Times. "But that's the way things are going in the industry. Basically, we all want to move to Copenhagen to get to do some extraordinary television."
Meanwhile, Alun Armstrong claimed that the cast had re-written parts of the script to improve them, "if the writer wasn't around".
Responding to the comments, writer Julian Simpson said: "Wait a second... *smells a rat* if certain actors re-wrote the scripts, how come they don't know the lines better?
"I was going to be writing today, instead I'm just going to hand the actors a pad and pen. I wish I'd learned this 15 years ago."
He continued: "A New Tricks I wrote and directed airs on Monday. I can tell you EXACTLY how much of it the actors wrote: not a f**king comma."
"The following week, Sarah Pinborough's episode is on. I directed that too. Cast contribution to script? Big fat zero.
"I also wrote and directed episode 10 of the new series. How much did the cast change? Are we sensing a pattern here...?"
He also retweeted a joke about the cast: "Dennis Waterman wants to go to Denmark to do some 'extraordinary TV.' You hear that sound? That's the Borgen writers howling with laughter."
Simpson later joked: "As has been widely reported, New Tricks cast members are leaving next series. Guess who's writing their exit eps... *finger guns*."
Fellow writer Lisa Holdsworth suggested that a new storyline on the show could involve "an unfortunate bus crash as they are all driven to the old people's home".
Commenting on the cast outburst, she said: "I'd like to say that it shocked me, but it didn't. W**kers."
New Tricks returns on Monday, August 27 at 9pm on BBC One. A tenth series is already planned for 2013.