The Archbishop of Canterbury is warning the chances of reaching a Brexit deal within the two-year deadline are 'infinitesimally small'.
Justin Welby said unless certain decisions were taken 'off the political table' the UK risked leaving the European Union without a trade deal.
A Remain voter, Welby has called for a cross-party commission to achieve consensus on Brexit negotiations and draw the 'poison' from the debate.
He warned of 'literally thousands of separate agreements' that need to be reached as well as the 'huge political decisions' such as membership of the single market and customs union.
'If each one of those has to be argued as a point of confidence on the floor of the House of Commons, the chance of getting this done in what's now roughly 18 months is infinitesimally small,' he told the BBC's Todayprogramme.
'Can the politicans not put at the front of their minds the needs of the United Kingdom to come out with a functional, working system for Brexit?'
He urged parliamentarians to 'agree that certain things are off the political table and will be decided separately in an expert commission'.
Welby was speaking from Sudan, where he led the inauguration of a new Anglican province on Sunday with Ezekiel Kondo Kumir Kuku the country's first archbishop.
It comes after he called for a commission to try and achieve agreement on Brexit.
Writing in the Mail on Sunday in June Welby said: 'Exit negotiations will be fierce and the differences on what we should aim for, and how, are very deep. They divide our politicians and our society.
'With a hung parliament, there is an understandable temptation for every difference to become a vote of confidence, a seeking of momentary advantage ahead of the next election.
'For that to happen would be a disaster if our negotiators, faced with the united determination of the EU, go into the room without confidence in their backing in the UK.
'It might turn us inwards and forfeit the opportunity to be a country the world admires and blesses for our generosity and vision.'
Welby said politics was 'rightly hard and tough' but proposed a commission chaired by a senior politician as a forum to achieve consensus.
'Recent events have highlighted the urgent need for a process of internal reconciliation, between regions, social groups, faiths and generations.
'The future of this country is not a zero-sum, winner take all, calculation but must rest on the reconciled common good arrived at through good debate and disagreement.'