WARSAW (AFP) -- Jaroslaw Kaczynski, who as a child co-starred with his identical twin Lech in a film called "The little hoodlums who tried to steal the moon", is preparing for a new double-act as prime minister of Poland alongside his president brother Lech.

Jaroslaw Kaczynski, head of the ruling conservative Law and Justice (PiS) Party, was set to become premier following the surprise announcement Friday by Prime Minister Kazimierz Marcinkiewicz that he would quit.

Holding the two most powerful state positions, the 57-year-old twins are now set to become the only political double act of its kind in the world.

Born in Warsaw, Jaroslaw and his twin were inseparable as children.

They both studied law at the University of Warsaw, were together in the anti-communist opposition and when the Solidarity trade union was born. Both were close to Solidarity founder and former Polish president, Lech Walesa, in the 1980s.

Each was elected senator in the country's first free elections after the fall of communism in 1989.

In 2001, they co-founded the conservative, Catholic PiS party, which scored the best result in Poland's general election last September.

Then in October, Lech scored a surprising win in the presidential election over Donald Tusk of the liberal, business-friendly Civic Platform (PO) Party.

Jaroslaw, who heads PiS, was considered the obvious choice for the post of prime minister after PiS finished first in the legislative vote.

But he demurred in favor of little-known economics expert Marcinkiewicz, a move many said was taken to protect his twin brother's chances in the presidential election.

Former Solidarity comrade-in-arms to the Kaczynskis, ex-president of Poland Lech Walesa said Friday: "Jaroslaw Kaczynski should have become prime minister straight away. Then, the situation would have been clearer from the start. "Now the two Kaczynskis will destabilize everything. We will have trouble picking up the pieces that will be left by their government."

Although co-workers have been known to have difficulty telling them apart, the twins are not totally identical.

Jaroslaw is single and lives with the twins' mother in Warsaw; Lech is married, with a daughter.

Lech has moles on his cheek and nose, and for a while had a moustache, making it easy to tell him apart from Jaroslaw who has always been clean-shaven.

"In political terms, we share the same opinions, the same patriotism as our parents, who fought in the resistance in World War II," Lech has said. He also highlighted one of the political advantages of having a twin. "You always have a colleague or boss you can count on."