Premature babies can actually feel pain and are not just displaying a reflex reaction to a stimulus, a team of doctors and scientists said on Tuesday.

Using brain scans of tiny babies born as early as 24 weeks after conception they found that during routine procedures such as obtaining a blood sample from a heel they feel pain.

"This is the first time we have actually measured pain activity in the human brain," said Professor Maria Fitzgerald, of University College London.

"It shows the specific information about pain is getting into the brains of these very young infants," she told Reuters.

Until now, information about pain in premature babies has been limited to physical expressions such as flinching or crunching the face.

But Fitzgerald said it has been difficult for researchers to interpret the significance of those reactions, which can also be triggered by something like a loud noise.

Using near-infrared spectroscopy, which measures blood levels and oxygenation in the brain, Fitzgerald and her team recorded activity in the brains of 18 premature babies born between 23-45 weeks from conception as nurses performed routine blood tests using a heel lance.

The scans showed pain information was being processed in the brain.

Because it has been difficult to measure pain in very tiny babies, treatment to relieve it has been sub-optimal, according to Fitzgerald, who reported the findings in The Journal of Neuroscience.

"Now that we have this scientific, objective measure of pain, we’ll be able to assess pain-relieving therapies much more precisely," she said.

"We’ll be able to see if they are actually working, or not."

Exactly when the so-called "pain pathways" in the brain begin to develop is not certain but scientists estimate between weeks 23 and 30.

Fitzgerald added that understanding and processing pain is something that is learned over a long period of time.

The sensitivity to pain in the babies in the study increased as they grew.

Fitzgerald said there is a possibility that because pain information is getting into their immature brains, which are still developing, it could change their response to pain later in life.

"It is certainly something we need to be aware of. It is another good reason for treating the pain and alleviating it at this very early stage when they are so vulnerable," she added.