The card was intended for a cellmate, but prison staff intervened and seized 2.3 grammes of amphetamine.

Mother of three Bridget Wilkinson had only been freed from the jail two weeks earlier.

The 43-year-old from Wakefield Road was found guilty of attempting to supply drugs to Jennifer Eiley at a trial in Stafford last month.

Sentencing her at Stafford Crown Court this week, Judge John Maxwell said a custodial sentence was unavoidable and sent her to prison for 15 months.

"You have been to jail four times and must have been fully aware of the implications of smuggling drugs into a prison," he said.

Wilkinson was arrested after officers at Drake Hall women's prison, near Stafford, intercepted the card, addressed to cellmate Eiley, and found the drugs – worth £80 in prison currency.

Judge Maxwell told Wilkinson: "You know perfectly well that when drugs get smuggled into prison the courts have no choice but to impose a custodial sentence.

"I can't avoid a prison sentence. The public would never accept that. The problem of drugs in prison is a serious one."

Defending, Nikki Peers said: "The drugs did not actually reach the person for whom they were intended. The amount of amphetamines was small with a purity of only five per cent.

"She was at the lowest level of the supply chain. She made no personal gain or profit.

"She has two daughters and a teenage son and is concerned about the effect her prison sentence will have on her family."

At her trial, prosecutor Paul Farrow said Wilkinson's DNA was found on a sticky substance behind the Valentine heart and on the envelope. Wilkinson told the jury that the card was given to her by a woman she did not know and who had turned up at her house.

"She wrote on the card and I put it in the envelope and sealed it," she said. "I had nothing to do with drugs and I never would."