The proposed canal would relieve the Bosphorus, the sole passage between the Mediterranean and Black Seas [EPA]

Turkey's leader has unveiled plans to build a major canal connecting two seas near Istanbul in an effort to relieve the congested Bosphorus strait.

The waterway would be built on the European side of the city, with dimensions of up to 50km long, 150m wide, a depth of 25m and would be large enough for supertankers to pass through.

"We have today embarked on the greatest project of the century," Recep Tayyip Erdogan, the Turkish prime minister, said as he announced the plans on Wednesday, which he called "crazy and magnificent".

"We are building the canal of the century," he said, "a project of such immense size that it can't be compared to Panama or Suez canals."

Connecting the Black and Marmara Seas the canal would be completed by 2023, the year Turkey celebrates its centenary of the founding of the Turkish republic.

"Istanbul will become a city with two peninsulas and an island," Erdogan said.

"The first peninsula already exists: the Asian side. The island will be formed between the canal and the Bosphorus when it cuts through the city, leaving a peninsula on the west."

Erdogan made the announcement a week into a campaign for parliamentary elections in June.

Kemal Kilicdaroglu, leader of the main opposition Republican People's Party (CHP), said it was a scheme to enrich the ruling AK Party supporters.

"This country needs men who think and produce, but not crazy men. This project is not about people. It's about making AK Party supporters rich," the CHP leader said.

Other plans for Istanbul

Erdogan, who served as mayor of Istanbul between 1994 and 1998, also has other plans to transform his home city.

In mid-April he announced plans to build two new towns on both sides of the city, while a tender to construct a third bridge over the Bosphorus will be held this summer.

A Turkish-Japanese consortium is currently building a rail tunnel under the Bosphorus to ease the congested traffic in Turkey's greatest city.

The 30km long Bosporus strait that bisects Istanbul is, in conjunction with the Dardanelles, the sole passage between the Mediterranean and the Black Sea and is heavily congested with tanker traffic from eastern Europe.

It has been the scene of ship accidents in the past and environmentalists warn a major disaster is waiting to happen.

Erdogan's AK Party is expected to secure a third term of single party rule when the country votes on June 12.

Opinion polls suggest a victory of similar margin to the 2007 result, when the AK scored 46.4 per cent of the vote.

Source: Agencies