A kidney "grown" in the laboratory has been transplanted into animals where it started to produce urine, US scientists say.

Similar techniques to make simple body parts have already been used in patients, but the kidney is one of the most complicated organs made so far.

A study, in the journal Nature Medicine, showed the engineered kidneys were less effective than natural ones.

But regenerative medicine researchers said the field had huge promise.

Kidneys filter the blood to remove waste and excess water. They are also the most in-demand organ for transplant, with long waiting lists.

The researchers' vision is to take an old kidney and strip it of all its old cells to leave a honeycomb-like scaffold. The kidney would then be rebuilt with cells taken from the patient.

This would have two major advantages over current organ transplants.

The tissue would match the patient, so they would not need a lifetime of drugs to suppress the immune system to prevent rejection.

It would also vastly increase the number of organs available for transplant. Most organs which are offered are rejected, but they could be used as templates for new ones.