The boy underwent a 90-minute operation at Great Ormond Street Hospital to remove the four-inch nail, nearly one inch of which was embedded in his brain.

Doctors are confident he will avoid brain damage. He is being kept under 24-hour observation, but is expected to leave hospital within days.

Surgeons carried out a procedure known as a craniotomy to remove the nail.

Small holes were drilled near the nail, which had penetrated the left frontal region of the brain.

The surgeons then cut between the holes to create a lid of bone that could be lifted back.

After the nail was removed, the bone was replaced, and the membranes, muscle and skin were sewn back into position.

Good progress

The child underwent surgery at Great Ormond Street Hospital

A hospital spokeswoman said on Monday that the boy, who has not been named, is making very good progress.

She said: "He is up and running about and doing extremely well.

"Anybody with a nail in their head is lucky to be alive."

A craniotomy is the standard procedure used in neurosurgery.

It is used to treat:

• Brain tumours
• Weaknesses in blood vessels
• Bleeding and blood clots caused by physical injury
• Damage to tissues covering the brain
• Severe nerve or facial pain

The front part of the brain, where the nail penetrated, has a role in controlling the personality, behaviour and the emotions.

It is also involved in planning, organising and problem solving.

Good chance of full recovery

Trevor Powell, a clinical psychologist working for West Berkshire Priority Care Services, said there was a good chance that a young person could make a full recovery from apparently serious brain injury.

"There is a possibility that an injury of this sourt could cause some minimal brain damage," he said.

"Memory and concentration could be impaired, as well as the ability to more than one thing at a time.

"But a younger person's brain is less set in its ways than somebody who is older. The neural pathways have not been laid down as rigidly, the brain is more plastic, and a younger person is more likely to make a better recovery than someone who is older."

Mr Powell said it was very difficult to predict how a brain injury would affect long-term function.