Following his departure, Mr Hall, aged 55, sold 250,000 shares. While his shares were placed at an undisclosed price by UBS, the sale will have made Mr Hall, who still owns some 150,000 shares in shares and options, a millionaire. He will be replaced by David Evans, Shield's finance director.
Mr Hall said that his decision to retire, first released in a bungled news release on Monday before it was quickly withdrawn, was no reflection on the potential of AFT, its big hope product.
Shares in the company rocketed from 130p earlier this year on hopes that its Activated Factor 12 product would replace cholesterol tests as the standard risk measure of heart disease. Mr Hall said discussions with several large companies, thought to include Abbott, to commercialise AFT were unaffected by his retirement.
"Talks are progressing. This has nothing to do with the way discussions are going and there is no rift on the board. I want to spend more time with my daughter," he said. "We are close to bringing off a commercial agreement for AFT. There is no way on earth that I wanted to prejudice the discussions by not announcing my retirement beforehand."
Mr Hall will stay on as adviser in any talks and emphasised that he retains 35 per cent of his shareholding.
Shield's shares hit a 919p high earlier this year before plunging to 500p on news that bungled handling of blood samples had spoiled data.
-Sameena AhmadReuse content