A new phone app could be a "game-changer" in the fight against child marriage in Bangladesh, where more than half of all girls are married before they are 18, children's charity Plan International said on Monday.

The impoverished South Asian nation has one of the world's highest rates of child marriage, according to UNICEF, despite laws that ban girls under 18 and men under 21 from marrying.

The mobile app being rolled out by Plan and the Bangladesh government aims to prevent it by allowing matchmakers, priests and officers who register marriages to verify the bride and groom's ages through a digital database.

"If we could get the people involved in the initial stages of marriage on side as well, then there would be no one to solemnize, no one to register and no one to arrange a marriage for a child," said Soumya Guha, a director at Plan Bangladesh.

"The app could be the game-changer that we need," he said, adding that it stopped 3,750 underage marriages during a six-month trial.

Campaigners say girls who marry young often drop out of school and face a greater risk of rape, domestic abuse and forced pregnancies, which may put their lives in danger.

Image: UNICEF

The app, which has an offline text messaging version for rural areas, gives the user access to a database that stores a unique identification number linked to the three documents.

When one of the numbers is entered, it shows "proceed" if the person is of legal age and a red "warning!" if not.

All marriages in Bangladesh must be legally registered within 30 days of the ceremony, but many are not.

A hard copy of a birth certificate, school leaving document or national identity card works as age proof, but often parents who want to marry off their children often forge them.

The charity is training 100,000 officiants about the ill effects of child marriage and how to use the app, which it hopes to roll out nationally by August.

"I believe this app will help us achieve the commitment by our honourable prime minister to eliminate child marriage before 2041," Muhammad Abdul Halim, a director general at the prime minister's office, told the Thomson Reuters Foundation.

However, Supreme Court lawyer Sara Hossain said more needed to be done to educate girls about their right to consent and plug legal loopholes.

"People might just avoid the registration because it is not required for validity of marriage and there is only a minor penalty for not registering. It's not a big thing," Hossain told the Thomson Reuters Foundation.

"We would be mistaken to think that something like this will be a magic bullet solution."