More than 40,000 women have not received "serious" information regarding their cervical cancer screenings.
NHS England said the affected women failed to get an invitation, reminder and result letters on time.
It blames the blunder on a "series of failings" made by Capita, the organisation contracted to provide GP back office services.
Capita said around 43,200 women were due to receive letters between January and June.
Each woman was due to receive a letter inviting them for an appointment followed by a reminder.
However, those affected only received one of these letters.
The British Medical Association's GP committee chair, Dr Richard Vautrey, described the blunder as an "incredibly serious situation".
He added: "It is frankly appalling that patients may now be at risk because of this gross error on the part of Capita.
"Some women will now be left extremely anxious because they have not received important correspondence, particularly letters about abnormal smear test results that need urgent follow up.
"This has been caused solely by Capita’s incompetence".
Explaining why the letters need to be sent on time, Dr Vautrey said: "We know that, because of the nature of this procedure, many patients are already reluctant to attend these appointments, and therefore reminder letters are crucial to provide encouragement and reinforce the importance of having a cervical smear test done".
In a statement, Capita apologised for the error.
They said: "Primary Care Support England (PCSE), the contract delivered by Capita on behalf of NHS England, is addressing an issue relating to issuing invitation and reminder letters regarding cervical screening correspondence.
"Letters are being sent to all women who have experienced a delay in receiving cervical screening correspondence.
"Additionally, there has been an issue relating to issuing results letters.
"The risk to women of this incident is low and there is no current evidence of harm, but Capita nevertheless apologises to both the NHS and to the women whose correspondence was delayed".
Most women's test results show that everything is normal, but for around one in 20 women the test shows some abnormal changes in the cells of the cervix.
In some cases, the abnormal cells need to be removed so they can't become cancerous.
Around 3,000 cases of cervical cancer are diagnosed each year in the UK.