The prime minister's resignation means the entire cabinet must step down, according to Peruvian law [EPA]

Salomon Lerner, Peru's prime minister, has stepped down after just five months in the job according to a resignation letter reportedly viewed by local and international media.

"With the sole purpose of giving you complete freedom to make any necessary adjustments in the cabinet, I offer you my irrevocable resignation"

- Salomon Lerner, Peru's PM

Lerner's resignation on Saturday would be the first ministerial change since President Ollanta Humala took office in late July. Lerner had been Humala's campaign manager and many observers considered him to be Humala's right-hand man.

Interior Minister Oscar Valdes, a former military officer who then worked in the private sector, was named the new prime minister. Valdes has seen his profile rise in recent weeks as the governmefnt worked to quell social unrest, especially in a restive northern mining area.

The decision by Lerner to step down automatically means the entire cabinet of Humala, a leftist former army officer, must also resign, according to Peruvian law.

President Humala, who cancelled a planned trip to Argentina to attend the inauguration of President Christina Kirchner, had no immediate comment.

'Complete freedom'

Peru's cabinet traditionally steps down each December to allow the country's president some leeway to reshuffle his line-up of ministers.

"With the sole purpose of giving you complete freedom to make any necessary adjustments in the cabinet, I offer you my irrevocable resignation," Lerner said in his letter to Humala.

Many of Peru's current ministers could keep their jobs under Lerner's successor, La Republica, seen as close to the government, reported on its website.

The departure of Lerner, a millionaire businessman before entering government, comes after Humala declared a state of emergency in a restive mining region in northern Peru in a bid to quell several days of labour unrest.

Workers in the Cajamarca department had gone on strike for more than 10 days in protest at what they fear will be adverse environmental impact from a planned $4.8bn gold and copper mining initiative.

Eclectic cabinet

In July, Humala became Peru's first leftist president in almost four decades, succeeding center-right leader Alan Garcia. He has pledged to confront the high poverty plaguing his Andean nation despite its solid economic growth.

Humala won a second-round run-off in June over Keiko Fujimori, daughter of Peru's disgraced former president Alberto Fujimori.

He brought together liberals and moderate leftists for his eclectic first cabinet, including leftist writer Rafael Roncagliolo as foreign minister and Lerner as his prime minister.

Source: Agencies