When professor Qian Nairong published his dictionary of the Shanghai dialect in 2007, he was in some ways documenting a dying language.

The number of people speaking the rapid-fire language — a badge of identity for residents of China's commercial capital of more than 20 million people — is shrinking.

As the government maintains a decades-old drive to promote Mandarin Chinese as the official language, banning dialects from media broadcasts and schools, many young people are unable to fluently speak — or fully understand — the native Shanghai tongue.

An influx of migrants from outside Shanghai and the city's drive to become more international have also combined to water down the local patois.

"A language is like a living thing, after it gets old, it must die," said Qian, a retired professor of language studies at Shanghai University. "People born in the 1990s cannot really speak Shanghai dialect."

Shanghai is not alone. China's southern province of Guangdong has announced plans requiring broadcasters to get special permission to use the Cantonese dialect in programs from March 1, causing a storm of controversy.

But there are signs that Shanghai's residents will not give up on their language that easily.

Shanghai comedian Zhou Libo has helped revitalise interest in the dialect with his witty routines, which often mock outsiders.

"The weakening of dialects means the weakening of local culture. Why must our children speak (Mandarin) Chinese? Shanghai people who cannot speak the Shanghai dialect. What stupidity!" he said on a talk show.

City buses have recently introduced announcements in Shanghainese, and there are plans for the metro system to follow suit to cater to older people, especially those with no formal education, who do not understand Mandarin.

Shanghai Airlines has just added an announcement about the city's attractions to market the unique features of Shanghai. But the airline had to train young flight attendants to pronounce the words.

Not long after the Communist Party took power in 1949, it made Mandarin Chinese the official language to promote unity.

The government has been unable to stamp out local dialects, but it has discouraged their use, barring them from the classroom in most cases.

Although millions of people — mainly those aged from their 40s — still speak the Shanghai dialect, the version Qian heard in the 1980s is now found only in the mouths of elderly people and in the city's distant suburbs.

Qian, 66, laments the disappearance of words and changes in pronunciation as Shanghainese borrows from dominant Mandarin.

"As soon as the sounds combine, a group of words change," he said.

Also lost to time are colourful curse words like "corpse floating on the river" and "ghost of an executed criminal" which were once common insults in the Shanghai dialect.

In the 1990s, the local government pulled radio and television broadcasts using the Shanghai dialect as part of a national campaign.

A popular Shanghai radio show "A Fu Gen" which featured discussions of current events was among the victims.

Xiao Ling, a host for the show, struggled for 10 years before a sympathetic official revived the program.

Xiao is one of only two hosts at the radio station who have formal broadcast training in the Shanghai dialect.

Last year, the program was forced to hold open auditions to find candidates with Shanghai language skills to fill open positions.

"My colleagues joke that we are giant pandas," said Xiao.

Shanghai's prestigious Tongji University organized a voluntary class in the dialect, after finding student volunteers were unable to communicate with elderly Shanghai people, teaching basic phrases like "nong hao" (hello).

Tongji also offers a class for foreign students.

"We thought it was good to have people from outside the organization and even outside the school to join the class," organiser Shen Yiwen said.