Authorities in Greece are struggling to put in place infrastructure to implement the deal signed by EU leaders and Turkey in Brussels to stem the flow of migrants.
From today, all "irregular migrants" who arrive in Greece will be sent back to Turkey.
However the process of the 'turn-backs', as they are known, will not actually begin for several weeks at least.
Speaking to Sky News, an official from the Greek government's crisis management office said the challenges were huge.
"If we had to do it today, we wouldn't be able to do it. There are things that have to be done before we are ready to implement a deal like this," Giorgos Kyritsis said.
"We are talking days in terms of the legal procedures. We have to make many legislation arrangements and then we have to make the infrastructure and that is a matter of weeks, not months."
Under the turn-back deal signed in Brussels on Friday, Turkey agreed to take back all irregular migrants who make the sea crossing to Greece. That includes refugees fleeing war.
To comply with international and EU law, the deal stipulates that each person to arrive in Greece will be given an interview.
If an individual chooses to claim asylum in Greece, and that claim is successful, they can stay in Greece.
But if they wanted to travel further to another EU country they would be refused and turned back to Turkey.
In Turkey they join the back of the queue of refugees eligible for resettlement in Europe.
Each person will be given the right to appeal the decision.
The European Commission has announced that implementing the scheme would involve the use of 4,000 personnel.
"The implementation of the agreement will require huge operational efforts from all involved, and most of all from Greece," an EU commission statement said.
"EU Member States agreed to provide Greece at short notice with the necessary means, including border guards, asylum experts and interpreters."
To implement the asylum process, 250 Greek asylum case workers and a further 400 asylum experts from the EU would be needed along with 400 interpreters.
The appeals process would involve 10 appeals committees, each with 30 members and headed by a judge with expertise in asylum law.
The turn-back process itself would involve the use of 1,500 police officers from around the EU as well as a further 1,000 other security staff or military personnel.
Greece is expected to provide the bulk of the personnel but other EU member countries are being asked to provide the rest.
"We are doing our best but after six years of severe austerity, the public sector here is understaffed, under-financed, has a lot of problems but we are doing our best," Giorgos Kyritsis from the Greek government said.
"We need to build more facilities to house these people, we have to feed them, we have to provide medical assistance. All this costs money. The Greek budget is very tight. We take money from the Greek budget because as for now, we didn't have any substantial aid from the EU," Mr Kyritsis added.
Sky News understands that the specifics of how people will actually be turned back have yet to be determined.
One option is to send all those who arrive on the Greek islands by ferry to the port of Kavala on the northern Greek mainland.
From there they can be taken by bus across the fortified land crossing between Greece and Turkey.
Another option is to take them back across the Aegean Sea in vessels provided by EU member states.
A source close to the process told Sky News the success of the whole scheme rests on whether EU countries deliver what they pledge both in terms of personnel and in accepting large numbers of refugees in the resettlement scheme from Turkey.
To date the EU has had a terrible track record in meeting pledges to ease the crisis.
Meanwhile, a number of legal experts and humanitarian organisations have said the deal breaks international law and is morally wrong.
Medecins Sans Frontieres said the deal was "ugly and illegal", while Amnesty International accused the EU of "turning its back on the refugee crisis".
Steve Peers, professor of EU and Human Rights law at the University of Essex, concluded: "Anyone with a legal qualification who signed off on this first sentence [of the deal document] should hang their head in shame.
"The... first sentence is a flagrant breach of EU and international law - but the rest of the paragraph then completely contradicts it."