The ban, demanded for decades by both Venice residents and environmentalists, will take effect on Aug. 1.

“The intervention could no longer be delayed,” Italy’s culture minister, Dario Franceschini, said in a statement.

In recent weeks, as cruise ships returned to Venice after the pause imposed by the pandemic, protesters in the city rallied on small boats and on the waterfront with “No big boats” flags. Last Sunday, they demonstrated during the Group of 20 summit for economic ministers that took place in the city, attracting international media attention.

“My heartbeat is so fast I could be having a heart attack,” said Tommaso Cacciari, an activist and spokesman for the No Big Ship Committee, responding to Tuesday’s announcement. “We have been fighting for 10 years, and now this victory feels almost unbelievable.”

In April, the government of Prime Minister Mario Draghi announced that it was planning to ban large cruise ships from the San Marco basin, the San Marco canal and the Giudecca canal, but no date for the ban was set. Also, the prohibition was conditioned on the building of a new port where tourists could disembark to visit the city, a project that could take years.

Tuesday’s decision removed that condition, so the ban could be enforced in weeks, not years.

Mr. Franceschini explained that the government had drafted the urgent decree to avoid “the real risk of the city being put on the blacklist of “World Heritage in Danger” sites established by UNESCO, the United Nations culture body.

In 2019, UNESCO warned Venice about the “damage caused by a steady stream of cruise ships.” Before a UNESCO World Heritage Committee beginning later this week that could have seen Venice added to the blacklist, the Italian government approved the decree making Venice’s waterways a national monument, a status usually given to artworks and historical buildings that puts the lagoon under enhanced state protection.

Over the last 10 years, Venice has been caught up in a clash between those representing the economic interests of cruise traffic — which employs thousands of people in the area — and others who want to protect a delicate environment from gigantic boats that disgorge tourists en masse.

The ban applies to ships that are either heavier than 25,000 tons, longer than 180 meters (about 590 feet), taller than 35 meters (about 115 feet), or that employ more than a set amount of fuel in maneuvering. The ban is such that even large yachts could be affected.

The government also decided to give power to the regional port authority to determine how five temporary docks can be built in Marghera, a nearby industrial port, while respecting maritime safety and environmental laws.

The intention to divert the cruise ships to the port of Marghera has raised eyebrows. The port is built for cargo ships and is not nearly as picturesque as the city’s lagoon. Moreover, the port’s channel is not large and deep enough for most cruise ships and would require major construction work.

Among the many projects considered by governments over the years, one envisioned a permanent passenger terminal at the Lido entrance to the lagoon. Activists considered that the best solution for the city and for the cruise industry.

Mr. Draghi’s cabinet also moved on Tuesday to establish compensation for sailing companies that will be affected by the ban and for other businesses connected to the cruise traffic inside the lagoon.

“It is a positive decision and could be the beginning of a new era,” said Francesco Galietti, national director for the Cruise Lines International Association. He added that the association has been asking for the temporary docking sites in Marghera since 2012.

The cruise industry is hoping, Mr. Galietti said, that the new docking sites would be ready in 2022, when tourists are expected to return en masse to cruises. This year, only 20 liners were expected to arrive in Venice.