"Unravelling decades of food law and regulations, let alone the labyrinthine Common Agricultural Policy (CAP), let alone generations of supply chains, let alone food tastes and consumers who are used to a food system based on EU security, is a very risky project," he said.
The Government's new Brexit unit "has terrifying responsibility", he suggested. "But what will it actually do? What expertise will it have? Will there be food specialists in there? Will it be dominated by big farm, big food and big supermarket interests? Who will champion consumer interests?"
Despite the efforts of Lang and other food academics and campaigners "there was next to no discussion of food" in the referendum debate, he added. "The powerful food industries shamefully kept their heads down and now are deeply anxious, as well they might be - they are in the front line. Food academics too are now in some disarray."
Yet since the vote "ironically, there has been a rapid rise of interest", said Lang. "Now more people 'get' it. If sterling drops in value, that means a country that imports 30 per cent of its food (much of it good news for health, fruit and vegetables) will have to pay more."
On the question of future farming subsidies, he said horticulture, which is "good news for health and heavily reliant on foreign-born labour, receives no subsidy yet should be vastly expanded".
He claimed: "Supply chains, consumer aspirations and market distortions have locked us into wasteful, unhealthy food cultures that have been normalised. If we really want to promote a more sustainable, equitable food system in and for the UK, now is the most important time to discuss issues and to speak out. History is littered with missed opportunities."
A paper Lang co-authored with Food Research Collaboration colleagues before the Brexit vote called for the replacement of the CAP with a "common sustainable food policy", saying an "overarching approach is necessary for the UK as well as Europe, regardless of the outcome of the country's referendum on EU membership" - one that would "integrate currently disparate food issues of carbon reduction, water conservation, food waste reduction, public health and consumer satisfaction alongside agricultural matters".
Lang said: "The problem is that policymakers are either too hesitant or dazzled by a belief that technology will resolve future food problems. They cannot. Food culture also needs to change."