Dr Wakefield's study sparked a major controversy
The doctor who controversially linked the MMR vaccine to autism has admitted a poor grasp of the medical ethics surrounding work on children.
Dr Andrew Wakefield is appearing before the General Medical Council charged with serious professional misconduct.
Among the allegations is a charge that he flouted ethical regulations by taking blood samples from children at a birthday party in return for money.
Experts stress that MMR poses no threat to children's health.
Dr Wakefield's work, which appeared in The Lancet, has been disowned by the journal.
At the GMC hearing on Friday, Dr Wakefield was asked if he really thought he could carry out research on children without any constraint.
He told the hearing that he had obtained parental consent - and thought that was enough to press ahead.
However, he added: "I'm perfectly willing to accept my understanding was wrong."
Dr Wakefield should have obtained clearance from an ethics committee for his work.
He admitted that he was not aware of "detailed guidance" on the treatment of children provided by the British Paediatric Association.
He said his colleague Professor John Walker-Smith, who also faces serious professional misconduct charges, was the expert in this field.
The 51-year-old, who is now working in the US, is accused of violating ethical guidelines, and of acting against the clinical interests of the children who took part in his trial.
He is also accused of acting dishonestly in failing to disclose to the Lancet that he was advising solicitors acting for parents who had alleged their children had been damaged by MMR.
The GMC case is not examining the safety of MMR, designed to protect against measles, mumps and rubella.
The publicity surrounding Dr Wakefield's original research lead to a public crisis of confidence in the vaccine, and a fall in uptake rates.
Since then a series of studies have repeatedly concluded that the vaccine is perfectly safe, and uptake rates have begun to climb again, although they are still under the recommended 95% in some places.
The number of confirmed cases of measles has risen from 56 in 1998 in England and Wales to about 1,000 last year.
Also facing professional misconduct charges is Professor Simon Murch.