“Nine were saved after four days at sea. The other 203 were swallowed by the waves,” UNHCR’s spokeswoman in Italy, Carlotta Sami, said on Twitter.
She called the situation a “horrible and enormous tragedy”.
On Monday, at least 29 migrants died after the inflatable boat carrying them overturned in high seas.
Seven were already dead when they were picked up near the Italian island of Lampedusa, and a further 22 succumbed to hypothermia after spending more than 18 hours on the open deck of the vessel which picked them up.
The International Organization for Migration says the two boats involved in the latest tragedy had departed from the Libyan coast on Saturday.
The IOM says that each boat was carrying more than 100 people when they capsized, probably on Monday.
The nine survivors all speak French, and are believed to be from West Africa.
Matthew Price, BBC News, Italy
There is no way of knowing for sure whether these 203 men, women, and presumably children, would have been saved if the former Italian search-and-rescue operation known as Mare Nostrum was still running.
But having spent a week on board an Italian navy frigate, I can be sure they would have done their utmost to save as many lives as possible.
The EU’s Triton border patrol is not designed to do that. It cannot pre-empt trouble in international waters – it can only act when lives are immediately at risk.
The Italian operation was set up differently. The naval crews knew they had one single purpose – to prevent death. Some time back, EU leaders pledged that not a single life would again be lost as a result of these large scale tragedies at sea.
Italy launched its Mare Nostrum search-and-rescue mission in October 2013, in response to a tragedy off Lampedusa in which 366 people died.
The aim of the mission was to look for ships carrying migrants that may have run into trouble off the Libyan coast, but it was disbanded in November 2014.
The European Union now runs a border control operation, called Triton, with fewer ships and a much smaller area of operations.
Ms Sami says the UNHCR had warned the EU that lives would be put at risk if search and rescue missions were not maintained in the area.
“At the moment we do not have a strong search and rescue capacity in the Mediterranean, while the influx of migrants and refugees is still very high,” she told Dutch TV on Tuesday, before news of the latest tragedy emerged.
The UNHCR says almost 3,500 people died attempting to cross the Mediterranean Sea to reach Europe in 2014, making it the world’s most dangerous sea crossing for migrants by some distance.
More than 200,000 people were rescued in the Mediterranean during the same period, many under the Mare Nostrum mission prior to its abolition.
In a speech before the European Parliament in November, Pope Francis called for a “united response to the question of migration”, warning that the Mediterranean could not be allowed to become a “vast cemetery”.
Wariye: Mohamed Hassan Daacad