Rome's ancient Colosseum is in a state of decay, archeologists say. (Gregorio Borgia/Associated Press)

The Italian government is looking for private sponsors to help it restore one of Rome's ancient landmarks — the Colosseum.

Ads will start appearing in major newspapers next week in a six-week appeal to raise money for a restoration that will cost an estimated $34 million.

The crumbling structure, the site of gladiator games in ancient Rome, has suffered badly in recent years and only 35 per cent of it is now open to the public.

In May, chunks of mortar plunged through a protective netting, raising the spectre of danger to visitors.

But the cash-strapped Italian government says it cannot afford to scrub and restore the Colosseum.

"We face a very big challenge that will be a national and international example of bringing together private and public funds," Rome Mayor Gianni Alemanno told a news conference Wednesday.

He said private funds had been used before, in the 1994 restoration of Michelangelo's Sistine Chapel.

Italian Culture Minister Sandro Bondi said restoration could begin in October and be finished by 2013.

The work would include restoration of the facade, modernizing the electrical, surveillance and lighting systems and creating a new visitors centre.

Sponsors would be allowed to advertise their contributions in a way "compatible with the value and decorum of the Colosseum," the Culture Ministry said in a statement.

After the restoration, some of the six million tourists who visit the Colosseum annually would have access to parts of the monument currently off-limits, including underground areas where gladiators and wild animals awaited their entrance into the arena.

The site would remain open to visitors while restoration work is completed.