A friend of Kate's told Britain's Grazia magazine: "Everyone has just been waiting for the point at which she would snap with Pete. She's been fixated on the fantasy of their rock 'n' roll romance, and for the past few months they've been living in this little bubble, oblivious to everyone else. But once again she's found that drugs mean more to Pete than she does.
"Kate likes the fact that Pete is this wild, rebellious character but for her there is always a line you cannot cross but for him that line just doesn't exist."
Earlier this week, Pete, 27, was fined £770 for five counts of possessing drugs. The charges relate to his arrest in April for possession of less than 2 grams each of heroin, cannabis and cocaine.
Kate, 32, is convinced if Pete carries on using illegal substances and getting arrested he will end up back in jail and she is not prepared to be a "prison wife".
He served a short sentence for burglary in 2003 after he broke into his ex-Libertines bandmate Carl Barat's flat.
The friend added: "Kate has to work out if she thinks it can ever work with Pete. She has already said she's not going to be some prison wife, queuing up to see him. She needs to decide whether he is too much trouble or if it's the trouble that she is addicted to."

In other news, Pete Doherty is said to be desperately upset after finding himself accidentally playing a leading role in a grim death riddle writes the Mirror UK

Mark Blanco, the actor who fell to his death on Sunday night, had earlier been thrown out of a party after ranting at the Babyshambles rocker.

Mark, 30, was found sprawled outside a flat in Whitechapel, East London, belonging to Pete's literary agent Paul Roundhill. He had apparently fallen 30ft.

Doherty, 27, left before officers arrived and is likely to be asked to make a statement to detectives.

The tragedy happened just 24 hours before the Babyshambles singer - who has a string of drug convictions - was fined £770 by magistrates for heroin possession.

Last night, toxicology tests were being carried out on Cambridge graduate Mr Blanco who was taken to hospital on Sunday morning and died on Monday. A post-mortem examination revealed he died from "severe head injuries".