Turkey may be hosting far fewer than the 2.7 million Syrian refugees declared in official figures, according to a controversial new claim.

An article in an online magazine by an aid worker and a former aid worker argues that many of those registered as living in Turkey may already have left for Europe, while others may have been registered several times. The claim comes as European leaders prepare to thrash out the final details of a deal with Turkey designed to prevent a repeat of the influx of people who arrived in Europe last year.

Ankara drove a hard bargain at an emergency summit in Brussels this week. In return for agreeing to allow all refugees and migrants arriving in EU countries to be sent back to Turkey, the nation’s Prime Minister, Ahmet Davutoglu, asked EU leaders to double a package of aid for Syrians in his country to €6bn (£4.7bn).

Questions about the actual numbers were raised in an article published on The Balkanist website under the headline “Turkey’s mysterious disappearing refugees”.

Written by an aid official based in southern Turkey, who works for an unnamed organisation, and a former humanitarian worker, it cites figures showing that 880,000 people arrived in Greece last year after crossing by sea from Turkey. In the same period, the number of Syrian refugees reported as living in Turkey rose from 1.5 million to 2.5 million. The current total claimed by Ankara stands at 2.7 million.

For that increase to be accurate, the authors say, 80,000 refugees must have crossed into Turkey from Syria every month last year, excluding anyone who went straight to Europe. However, from about March last year it has become much harder for Syrians to cross into Turkey, as large sections of the border have been closed.

The article notes that the numbers of Syrians living in official government camps rose slightly in 2015 from 230,000 to 270,000.

The authors warn that the official Turkish figures are “being accepted uncritically by aid groups and government officials” when they should be treated with more caution – especially when they form the basis of frantic EU negotiations.

A senior Turkish official did not deny that the figure for Syrians in Turkey could be too high, but said that it was “difficult to tell”.

Refugee crisis - in pictures Show all 27 1 /27 Refugee crisis - in pictures Refugee crisis - in pictures A child looks through the fence at the Moria detention camp for migrants and refugees at the island of Lesbos on May 24, 2016. AFP/Getty Images Refugee crisis - in pictures Ahmad Zarour, 32, from Syria, reacts after his rescue by MOAS (Migrant Offshore Aid Station) while attempting to reach the Greek island of Agathonisi, Dodecanese, southeastern Agean Sea Refugee crisis - in pictures Syrian migrants holding life vests gather onto a pebble beach in the Yesil liman district of Canakkale, northwestern Turkey, after being stopped by Turkish police in their attempt to reach the Greek island of Lesbos on 29 January 2016. 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All other nationalities are deemed economic migrants and told to turn back. Macedonia has finished building a fence on its frontier with Greece becoming the latest country in Europe to build a border barrier aimed at checking the flow of refugees Refugee crisis - in pictures A father and his child wait after being caught by Turkish gendarme on 27 January 2016 at Canakkale's Kucukkuyu district Refugee crisis - in pictures Migrants make hand signals as they arrive into the southern Spanish port of Malaga on 27 January, 2016 after an inflatable boat carrying 55 Africans, seven of them women and six chidren, was rescued by the Spanish coast guard off the Spanish coast. Refugee crisis - in pictures A refugee holds two children as dozens arrive on an overcrowded boat on the Greek island of Lesbos Refugee crisis - in pictures A child, covered by emergency blankets, reacts as she arrives, with other refugees and migrants, on the Greek island of Lesbos, At least five migrants including three children, died after four boats sank between Turkey and Greece, as rescue workers searched the sea for dozens more, the Greek coastguard said Refugee crisis - in pictures Migrants wait under outside the Moria registration camp on the Lesbos. 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Macedonia is a key transit country in the Balkans migration route into the EU, with thousands of asylum seekers - many of them from Syria, Afghanistan, Iraq and Somalia - entering the country every day Getty Images Refugee crisis - in pictures An aerial picture shows the "New Jungle" refugee camp where some 3,500 people live while they attempt to enter Britain, near the port of Calais, northern France Getty Images Refugee crisis - in pictures A Syrian girl reacts as she helped by a volunteer upon her arrival from Turkey on the Greek island of Lesbos, after having crossed the Aegean Sea EPA Refugee crisis - in pictures Refugees arrive by boat on the Greek island of Lesbos after crossing the Aegean sea from Turkey Getty Images Refugee crisis - in pictures Beds ready for use for migrants and refugees are prepared at a processing center on January 27, 2016 in Passau, Germany. 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He told The Independent: “We don’t know if 100 per cent of Syrians currently in Turkey are registered, even though the authorities have been trying very hard to register everyone. At the same time it is unclear how many people left – how many people drowned, or ended up in Europe.” He suggested that the numbers in Europe could also be inaccurate because people who travelled to Germany or Sweden crossed many national borders and were re-registered and reported every time they did so.

All those arriving in Greece from Turkey are supposed to be registered, raising the prospect that they could, in theory, be crossed off against the list of names of refugees in Turkey. Frontex, the EU border agency, has sent hundreds of officers, seconded from police forces across Europe, to help the Greek authorities record all new arrivals.

However, a spokesman for the agency said that information about individuals was passed only to the Greek authorities and could not be shared more widely because of data protection laws.