With only four nuns left, southern Spain's historic Corazon Eucaristico de Jesus convent is on the brink of closure.

Now its prioress is calling on sisters everywhere to take a leap of faith, come up to the sanctuary in the mountaintop city of Ronda and stay on for a while to keep it open.

"We will do everything we have to do, but we know at the end it's God's wish that will prevail," Prioress Jennifer del Corazon de Jesus told Reuters.

The Roman Catholic nuns are hoping to celebrate the 100th anniversary of their move to the site in October.

But Jennifer says they may not last that long. Under Vatican rules, convents with fewer than six members cannot remain open.

"If in your convent you've got enough manpower and you would like to help us out for a year or two years, I know God will bless you and I'm sure our Holy Mother will give you a hand," said the 63-year-old who hails from Gibraltar.

The building dates back to the 16th century and houses the hand of St Teresa of Avila, kept inside a silver gauntlet decorated with stones. Pilgrims come to the site, many of them believing the relic can help with fertility issues.

The four nuns belong to an Order of Discalced Carmelites - discalced meaning barefoot or wearing sandals. They subordinate their lives to prayer and devotion, according to the convent's website.

They are primarily looking for nuns from that order. "If any nun should want to come they must show clear signs of having a Carmelite vocation," Jennifer said.

There were 13 nuns there when she arrived 39 years ago but most have since died. The oldest surviving inhabitant has lived there for six decades.

If the convent shuts, they would have to move to another convent.

"It's going to be very hard" to leave, Jennifer said. "It will affect us, naturally it will."