Andrew Wakefield, the researcher who sparked the MMR scare with a paper in The Lancet six years ago, did not cover up his links with the Legal Aid Board, it emerged yesterday.

Dr Wakefield was accused at the weekend of failing to disclose the conflict of interest over his research at the Royal Free Hospital in London, suggesting a possible link between the MMR vaccination and bowel disease and autism, which has led tens of thousands of parents to boycott the triple vaccination.

But he did reveal his links with the Legal Aid Board in a letter published in The Lancet on 2 May 1998, less than three months after his original research paper.

Responding to critics of his study who suggested it was suffering from "litigation bias", he admitted that he had been asked to "evaluate a small number of children by the Legal Aid Board". But he insisted that the children had been referred to him through normal channels and there had been no bias in the way they were selected.

The disclosure raises questions about why The Lancet did not repudiate the study at the time on the grounds that Dr Wakefield's involvement with the Legal Aid Board represented a conflict of interest. It only did so when the same allegations were put to it last week by The Sunday Times, almost six years later.

A four-month investigation by the newspaper revealed that Dr Wakefield had received £55,000 from the Legal Aid Board while working on the Lancet study in preparation for a possible claim against the vaccine manufacturers. At least four of the 12 children in the Lancet study were also included in the study for the Legal Aid Board. The money was paid into his research fund.

Richard Horton, editor of The Lancet, said at the weekend that Dr Wakefield's failure to disclose the conflict of interest left his research paper "fatally flawed". But the letter published in The Lancet in May 1998 made Dr Wakefield's links with the Legal Aid Board clear. In the letter, responding to critics of his paper published three months earlier, he wrote: "Only one author (AJW) has agreed to help evaluate a small number of these children on behalf of the Legal Aid Board." AJW are Dr Wakefield's initials.

The Lancet declined to comment on the letter yesterday but a spokesman for a medical publisher suggested it did not amount to a full disclosure because it came three months after publication of the paper, made an indirect reference to the legal aid study and did not mention any payment.