The Foreign Office said in a statement: "We are in contact with the Spanish authorities responding to flooding in Majorca, and urgently seeking updates from them".
Spain's meteorological office said the neighbouring islands of Ibiza and Formentera were now on high alert as the rain and storms moved there.
Sant Llorenc, which is about 60 kilometres east of Mallorca's capital, Palma, is where a dry creek overflowed amid heavy downpours, creating a strong current of water and mud that buried cars and tore trees along the way, according to witness accounts.
Two more bodies were recovered in the town of S'illot, an emergency services spokeswoman who wasn't authorized to be named in media reports told The Associated Press.
"My honest condolences to the relatives of the deceased and injured by the severe floods in San Llorenç".
Images posted on social media showed cars being swept along narrow streets by torrents of brown water.
The regional government called an emergency meeting, and authorities said rescue workers and military units had been sent to the area to help.
At least three towns closed schools and hundreds of people were sheltered in sport facilities and a local horse racing track, authorities said.
The Spanish parliament held a tribute to the victims in the form of a minute's silence.
"In just two hours, some 180 litres of rain fell and we realized that we could not control the water", she said.
"What is most urgent right now is to find missing people and address the concerns of their relatives and all neighbours", Prime Minister Pedro Sanchez said during a visit to the disaster zone.