The firm of Silverglate and Good said: "A slower, more deliberative and ultimately fairer investigation would not have resulted in this indictment." Several legal experts said it was unusual and manslaughter was a more likely charge in such cases.
Ms Woodward, 19, was arrested in the United States last month after nine- month-old Matthew Eappen died from a brain haemorrhage. Doctors said the baby's injuries were consistent with him being shaken so hard that his brain had smashed repeatedly against the inside of his skull. A post- mortem examination showed he had suffered a fractured skull, caused by a "blunt trauma wound" consistent with being thrown against a wall.
Ms Woodward, who had been working for the baby's family in Newton near Boston, Massachusetts, after taking her A-levels, was arrested and held in custody. The decision to charge her with murder was made by 22 grand jury members on Wednesday.
Several legal experts said yesterday that the first-degree murder charge, which carries an automatic penalty of life in prison without possibility of parole if convicted, was uncommon.
Joseph Balliro, a prominent criminal defence lawyer in Boston, said: "First-degree murder requires premeditation and malice aforethought. In other words, the District Attorney is going to have to persuade a jury that this girl intended to kill this kid."
Stephen Lyons, another Boston lawyer, said the massive amount of publicity the case had generated both in the US and Europe may have contributed to the decision. "The intense publicity has put a great deal of pressure on prosecutors to do what they think the public wants in this case," he said.
The baby's parents, Sunil Eappen, 30, an anaesthesiologist, and his wife, Deborah, 31, an ophthalmologist, have not commented on the case. But it has shocked American parents and raised questions about who cares for their children. Like many of the other 12,000 au pairs in the US, Ms Woodward was looking after Matthew and his older brother, Brendan, before going to university.
Her mother, Sue, told ITN that her daughter had not abused the child. "She can think of nothing she did that day that would cause Matthew to have any sort of seizure at all. She took very good care of him and did everything she could to help him when he was in difficulties."
Her daughter was making the best of a bad situation, Mrs Woodward added. "She realises she has to go through this process in order to prove her innocence."
But critics claim the case highlights a problem with lack of training and screening for au pairs. The young women, who are typically in their teens or early 20s and have no qualifications, are often unprepared for the work of caring for children.
In at least two cases in the US, secret cameras set up by suspicious parents have shown nannies battering the babies they were supposed to be looking after. Spying on nannies has produced its own business - closed- circuit cameras are now available in teddy bears and burglar alarm sensors.
EF Au Pair of Cambridge helped place Ms Woodward with the Eappens. It has been paying for her defence. In a statement, the agency said: "We support Louise Woodward's right to a fair trial and hope that the truth will be uncovered and justice will be served."