The BBC has obtained evidence casting doubt on the Greek coastguard's account of Wednesday's migrant shipwreck in which hundreds are feared to have died.

Analysis of the movement of other ships in the area suggests the overcrowded fishing vessel was not moving for at least seven hours before it capsized.

The coastguard still claims that during these hours the boat was on a course to Italy and not in need of rescue.

Greek authorities have not yet responded to the BBC's findings.

At least 78 people are known to have died, but the UN says up to 500 are still missing.

The UN has called for an investigation into Greece's handling of the disaster, amid claims more action should have been taken earlier to initiate a full-scale rescue attempt.

Greek officials maintain those on board said they did not want help and were not in danger until just before their boat sank.

The BBC has obtained a computer animation of tracking data provided by MarineTraffic, a maritime analytics platform.

Their data shows hours of activity focused on a small, specific area where the migrant boat later sank, casting doubt on the official claim it had no problems with its navigation.

The fishing boat had no tracker so is not shown on the map. Neither are coastguard and military vessels which do not have to share their location.

Frontex, the EU's border force, says it first spotted the migrant boat at around 08:00 GMT on Tuesday and informed the Greek authorities.

Alarm Phone, an emergency hotline for migrants in trouble at sea, say they received a call at 12:17 GMT saying the boat was in distress.

We have used video and photographs authenticated by BBC Verify, as well as court records and shipping logs, to analyse the movement of vessels in the area in the following hours.

The Marine Traffic animation shows a ship called the Lucky Sailor abruptly turning north at 15:00 GMT.

The owner of the Lucky Sailor gave the BBC their timeline of events and confirmed it had been asked by the Coastguard to approach the migrant boat and give food and water.

About half-an-hour later at 15:35 GMT, the coastguard helicopter found the migrant boat. Authorities have continued to claim it was on a steady course at the time.

But two-and-a-half hours later at around 18:00 GMT, another vessel, the Faithful Warrior, travelled to the same area and also gave supplies to the boat.

The owners of Faithful Warrior referred us to the investigating authorities.

Video has emerged - reportedly shot from the Faithful Warrior - claiming to show supplies being delivered to the migrant ship via a rope in the water. No other ships can be seen.

BBC Verify checked it and found the vessel - which is not moving in the footage - matched the shape of the migrant ship seen in photos and the weather conditions were a match for those reported at the time. It's not known exactly when this video was filmed.

Between 19:40 until 22:40, Greek officials originally claimed the boat was keeping a "steady course and speed".

Their initial statement claimed they observed from a discreet distance, but a close-up image they later published - from this time-period - suggests the migrant boat is not going anywhere.

A government spokesperson later said the coastguard had attempted to board the boat to assess the danger but that those on board removed a rope that had been attached and did not want help.

All of the shipping activity of the previous seven hours was focused around one specific spot, suggesting the migrant boat had hardly moved.

The scale of the animated map suggests it travelled less than a few nautical miles, which may be expected of a stricken vessel buffeted by the wind and the waves in the deepest part of the Mediterranean Sea.

The actions of people in distress, rocking the vessel, would also have contributed any movement.

During this time period, Greek officials have insisted it was not in trouble and was instead safely on its way to Italy and so the coastguard didn't attempt a rescue.

At 23:00, the boat sank with hundreds on board and the tracking animation shows a frenzy of ships coming to help.

This included the Celebrity Beyond from which footage of the aftermath of the disaster was filmed and later sent to the BBC.

A luxury yacht, the Mayan Queen, is then instructed to help take some of the 104 survivors ashore.

Those rescued reached the safety at the port of Kalamata but left behind a series of troubling questions about the whole Greek response.

Additional reporting by Josh Cheetham, Daniele Palumbo and Frey Lindsay.