The Prime Minister has offered to provide more help to Greece as it struggles to return migrants to Turkey.
David Cameron told Greek prime minister Alexis Tsipras that additional support will be available in the coming weeks, after the EU-Turkey agreement to tackle the migrant crisis took effect on Monday.
Greek authorities have warned the operation is threatened by a shortage of personnel.
A Downing Street spokesman said Mr Cameron spoke to Mr Tsipras on the phone on Tuesday afternoon.
He said: "The Prime Minister welcomed the start the Greek authorities had made to returning migrants to Turkey under the agreement, and reiterated the importance of breaking the link between people making the dangerous crossing of the Mediterranean and securing settlement in Europe.
"Prime minister Tsipras said that the flow of migrants had reduced substantially, and expressed gratitude for the UK contribution of personnel and expertise to help implement the agreement.
"The Prime Minister confirmed that additional UK personnel would be offered to Greece over the coming weeks to provide additional support."
Under the EU-Turkey agreement, those arriving on Greek islands from the nearby Turkish coast on or after March 20 are eligible for deportation if they do not apply for asylum or their application is rejected or inadmissible.
Greece began on Monday to deport migrants but Greek authorities told state TV that no transfers were planned on Tuesday after thousands of people held in migrant detention camps applied for asylum.
Maria Stavropoulou, director of Greece's Asylum Service, said it could result in a slow-down in the rate of returns under the deal.
She said o nly 30 of 400 migration officers from other EU countries have arrived in Greece so far to help with processing.
Giorgos Kyritsis, a spokesman for the Greek government's refugee crisis committee, told The Associated Press on Monday that Frontex, the EU's border management agency which is responsible for implementing the deal, only has 200 officers in place to accompany the deported migrants, but almost none of the other personnel that would facilitate screening those who apply for asylum.
More than 52,000 people have been trapped in Greece after Balkan and European countries shut their land borders to refugees and other migrants, and the EU forged the deal with Turkey.
Last year more than a million people entered the EU by making the short sea crossing from Turkey to the Greek islands.