Lawson, who testified for the prosecution at the trial of her two former personal assistants, admitted she had used cocaine but denied allegations she was a regular user.
Police said an a statement nothing that has come to light so far is adequate to bring charges against Lawson, CNN reported.
The Crown Prosecution Service and police officers "will determine an appropriate way forward," the agency said.
"The Senior Investigating Officer received legal advice that the witness's admissions did not by themselves provide sufficient evidence to bring charges," the statement said.
Elisabetta and Francesca Grillo were acquitted of defrauding Lawson and her ex-husband, Charles Saatchi, of more than a million dollars. While the sisters and Saatchi testified they had not witnessed Lawson using drugs, the Grillos said they had seen what appeared to be signs of heavy drug use.
Lawson released a statement Friday saying that during the trial she tried to make sure the CPS knew there was a "sustained background campaign deliberately designed to destroy my reputation."
"Over the three week trial the jury was faced with a ridiculous sideshow of false allegations about drug use which made focus on the actual criminal trial impossible," Lawson said.
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