TO PROVIDE an effective defense, lawyers must know that conversations with clients won’t be shared with prosecutors. That’s why the attorney-client privilege is critical and why federal prisons typically disallow content monitoring of prison visits, mail and secure telephone calls between inmates and their lawyers.

The introduction of prison e-mail has threatened this basic privilege. The New York Times reported last month that attorney-client e-mails are being read and used in court against defendants. In Brooklyn, “prosecutors have said that they intend to read such emails in almost every case,” according to the Times.

In one instance, the government gathered more than 12,000 pages of prison e-mails from former Pennsylvania state senator Vincent J. Fumo for use in his resentencing. Mr. Fumo, who was in jail for 137 counts of conspiracy, fraud and other charges, had been e-mailing his lawyer about a court decision and his plans. He was sentenced to six more months in jail.

These e-mails might have contained defense strategies, potentially giving prosecutors an unfair advantage in court. They might also have included incriminating evidence that was supposed to be shared only with his attorney. Prosecutors would rightly be denied access to such content if it came in a letter or secure phone call.

Some have argued that prisoners are warned that e-mails are monitored — meaning, as one judge put it, that prisoners “had no expectation of privacy.” Another judge, among some who have ruled in favor of prosecutors on this question, wrote that “the government’s policy does not ‘unreasonably interfere’ with . . . [the] ability to consult his counsel.”

If other forms of communication are unmonitored, then why not use them? Simple answer: Because they’re burdensome and impede timely access to counsel. Unmonitored calls can take weeks to request. Privileged mail is sometimes delivered two weeks after it’s sent, and frequent in-person visits are impractical for busy defense lawyers in large states. E-mails are quick, and from a security standpoint, safer than physical mail since there’s no risk of contraband being sneaked in.

A few offices, including in Brooklyn, once deployed “taint teams” to put aside sensitive e-mails but later eliminated them due to budget cuts. While understandable, cost is no excuse for breaking attorney-client privilege.

The Federal Bureau of Prisons can implement a lasting and universal solution. With a modest investment, it can monitor most e-mail traffic for security purposes but create a separate channel for attorney-client e-mails. This would give attorney-client correspondence the same protection as attorney-client phone calls.

Prosecutors are taking advantage of the gap between law and technology to gain an upper hand over defendants. That’s unacceptable — and the Justice Department should end the practice.