They were one of the world's most famous couples, who lived lives of power and glory – but who spent their last hours in despair and confusion. Now, more than 2,000 years since Antony and Cleopatra walked the earth, historians believe they may finally have solved the riddle of their last hours together.

A team of Greek marine archaeologists who have spent years conducting underwater excavations off the coast of Alexandria in Egypt have unearthed a giant granite threshold to a door that they believe was once the entrance to a magnificent mausoleum that Cleopatra VII, queen of the Egyptians, had built for herself shortly before her death.

They believe the 15-tonne antiquity would have held a seven metre-high door so heavy that it would have prevented the queen from consoling her Roman lover before he died, reputedly in 30BC.

"As soon as I saw it, I thought we are in the presence of a very special piece of a very special door," Harry Tzalas, the historian who heads the Greek mission, said. "There was no way that such a heavy piece, with fittings for double hinges and double doors, could have moved with the waves so there was no doubt in my mind that it belonged to the mausoleum. Like Macedonian tomb doors, when it closed, it closed for good."

Tzalas believes the discovery of the threshold sheds new light on an element of the couple's dying hours which has long eluded historians.

In the first century AD the Greek historian Plutarch wrote that Mark Antony, after being wrongly informed that Cleopatra had killed herself, had tried to take his own life. When the dying general expressed his wish to pass away alongside his mistress, who was hiding inside the mausoleum with her ladies-in-waiting, he was "hoisted with chains and ropes" to the building's upper floor so that he could be brought in to the building through a window.

Plutarch wrote, "when closed the [mausoleum's] door mechanism could not open again". The discovery in the Mediterranean Sea of such huge pieces of masonry at the entrance to what is believed to be the mausoleum would explain the historian's line. Tzalas said: "For years, archaeologists have wondered what Plutarch, a very reliable historian, meant by that. And now, finally, I think we have the answer.

"Allowing a dying man to be hoisted on ropes was not a very nice, or comforting thing to do, but Cleopatra couldn't do otherwise. She was there only with females and they simply couldn't open such a heavy door."

The threshold, part of the sunken palace complex in which Cleopatra is believed to have died, was discovered recently at a depth of eight metres but only revealed this week. It has yet to be brought to the surface.

The archaeologists have also recovered a nine-tonne granite block which they believe formed part of a portico belonging to the adjoining temple of Isis Lochias. "We believe it was part of the complex surrounding Cleopatra's palace," said Zahi Hawas, Egypt's top archaeologist. "This is an important part of Alexandria's history and brings us closer to knowing more about the ancient city."

According to Plutarch, who based his accounts largely on eyewitness testimonies, Antony died within seconds of laying eyes on his beloved queen and mother of his children.

Cleopatra, the most powerful woman of her day and Egypt's most fabled ruler, is believed to have taken her own life just days later, legend has it with the aid of an asp.