He also said he was "not convinced" that introducing a system presuming people had consented to donate, unless they carried cards opting out, would solve the organ shortage.
The Liberal Democrat health spokesman Dr Evan Harris, opening a short debate, said the "racist" donation row and the case of the 15-year-old girl forced to have a heart transplant highlighted the issue's importance.
He called on the Government to consider changes to the system if evidence suggested that the organ shortage could be reduced. One proposal was a review of families' right to veto express wishes to donate organs made by relatives before they died.
Dr Harris (Oxford West and Abingdon) said: "There is a question about whether the law is clear in this area and certainly the British Medical Association have long had policy questioning whether it's appropriate for ... the person whose organs are in question, whether the wishes of that person should be overruled by relatives."
He also urged a fresh look at the system of "presumed consent" and argued it should replace the current "opt-in" system of donor cards. People would then be presumed to be giving their consent unless they registered that they did not.
He said that about 70 per cent of the population had expressed a willingness to donate but only 17 to 18 per cent carried cards. There was a "chunk of people in the middle whose views are not known, where the majority are likely to be in favour" and relatives are asked to "second-guess at a very difficult time" their loved ones' views.
Earlier, outside the House, Dr Harris commented on the case of child M - who had her wishes not to have a life-saving heart transplant overturned in court.
"It's never an easy call for a judge to make but in this case I think that we must welcome the decision to save the life of this critically ill girl," he said.Reuse content