The Green Party is calling for the Home Office to be scrapped in a radical shake-up of immigration policy.
Co-leader Jonathan Bartley wants a new Ministry of Sanctuary to be set up to oversee a "fairer system" which includes the closure of all migration detention centres.
Speaking to his party's conference in Newport, he said there must be a decisive "break from business as usual" and a focus on tackling injustices that led to the Brexit vote.
Mr Bartley, who shares the leadership with Sian Berry, said the Home Office should be scrapped and be shared by two new departments including the Ministry of Sanctuary and the Ministry of the Interior.
The Ministry of Sanctuary would be in charge of immigration policy and the Ministry of the Interior would be responsible for law and order.
He said: "There is an imperative to fight not only the threat of Brexit but the conditions that have brought us Brexit.
"We will fight to remain in Europe and fight to transform Britain.
"This can be a new start. We need a decisive break from business as usual, and we are ready to make the leap.
"The Green Party has always been on the right side of history. The time is now to shape our future."
The party opposes Brexit and supports another referendum.
Although it only has one MP, it performed strongly at the European elections.
Mr Bartley is also calling for future laws passed by Parliament to be measured against their environmental impact on the next generation.
Co-leader Sian Berry will address the three-day event on Sunday.
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