In some cases, the hike in BP reading was enough to tip a patient over the threshold for needing treatment.
The difference may be because patients feel more anxious when they see a doctor - the white coat effect, say the University of Exeter researchers.
Their work is published at BJGP.org.
The researchers studied more than 1,000 patients whose BP readings had been taken by both doctors and nurses at the same visit.
Lead researcher Dr Christopher Clark said the study findings suggested doctors might not be best placed to monitor blood pressure.
He said: "Doctors should continue to measure blood pressure as part of the assessment of an ill patient or a routine check-up, but not where clinical decisions on blood pressure treatment depend on the outcome.
"The difference we noted is enough to tip some patients over the threshold for treatment for high blood pressure, and unnecessary medication can lead to unwanted side-effects."
Blood pressure can fluctuate throughout the day. Having one raised reading does not necessarily mean you have high blood pressure. Feeling anxious or stressed when you visit your GP can raise your blood pressure, the NHS advises.
Increasingly, to get more accurate readings, doctors are giving patients portable testing kits that measure blood pressure at home over the course of a day and night.
Spotting and treating high blood pressure is important to lower the risk of future complications such as heart attacks and stroke.
Katharine Jenner, of Blood Pressure UK, said: "Many people feel slightly anxious when going to see a doctor, even though having a blood pressure check is simple, quick and painless, it is interesting to see it quantified in this study."
She said guidelines recommended home monitoring of blood pressure as well as in the clinic.
Next time you visit your GP to have your blood pressure (BP) checked, you may want to ask the nurse to do it, say researchers who have found that doctors routinely record higher levels.