A British prisoner is among five inmates who escaped from a high-security prison in Portugal.

Authorities are searching for Mark Cameron Roscaleer and four other prisoners after they broke out of Vale de Judeus prison, located about 43 miles north of Lisbon, on Saturday.

The escapees scaled the prison wall using a ladder, with the aid of "external help," according to the Portuguese prison service.

Roscaleer is serving a nine-year sentence for kidnapping and robbery. Vale de Judeus, a high-security facility with a capacity of 560 inmates, confirmed that the escape occurred at 10am local time on Saturday.

In addition to Roscaleer, the group includes two Portuguese men, an Argentinian, and a Georgian.

The Portuguese escapees were serving 25-year sentences for crimes including drug trafficking, criminal association, theft, robbery, and kidnapping.

The others had been convicted of offences such as theft, kidnapping, and robbery.

Portugal's national prison guards union, Sindicato Nacional do Corpo da Guarda Prisional (SNCGP), shared the prisoners' mugshots on social media.

Frederico Morais, head of the union, warned that the escapees are highly dangerous and advised the public not to approach them.

He also pointed out that the escape was made possible by a shortage of prison guards, which left the surveillance towers unmanned, as reported by Diario de Noticias.

The prison service confirmed that the inmates used a ladder provided by external accomplices to scale the wall and flee the prison.

Portuguese police have sought international cooperation in the search for the escapees.