The official guidance means the salutation “evenin’ all”, which marked the start of each episode of Dixon of Dock Green, could be under threat.

The instructions form part of lengthy guidelines issued by police forces and fire services across the UK on what language their staff should use. Critics have accused the guides of “lacking common sense”.

Other words now discouraged include, “businessman”, “housewives” and “child”, which the organisations argue have negative connotations and could cause offence.

Confusingly, staff are also barred from using the word “homosexual”, for which they are instructed to use the term “gay”, while they are warned against using the phrase “straight”, and told to say “heterosexual”.

The instructions have emerged in response to a Freedom of Information request to police forces and fire services about the guidance they give their staff on their use of language. One force urging caution over the use of “evening”, is Warwickshire Police.

Under a section entitled “Communication, Some Do’s & Don’ts”, in its “Policing Our Communities” handbook, it gives advice to officers on communicating with people from different ethnic groups. It states: “Don’t assume those words for the time of day, such as afternoon or evening have the same meaning.”

A spokesman added: “Terms such as ‘afternoon’ and ‘evening’ are somewhat subjective in meaning and can vary according to a person’s culture or nationality. In many cultures the term evening is linked to time of day when people have their main meal of the day.

“In some countries including the UK, the evening meal time is traditionally thought of as being around 5-7pm but this might be different say for a family say from America who might have their main meal earlier and thus for them ‘evening ‘ may be an earlier time.

“The point is there is an element of subjectivity leading to a variation between cultures that we need to be aware of – taking steps as far as possible to ensure our communication is effective in serving the public.”

A number of organisations, among them Essex Police and Northern Ireland Fire and Rescue Service, now instruct staff to avoid the phrases “child, youth or youngster”.

The 52-page guide used by both organisations states that such phrases could have “connotations of inexperience, impetuosity, and unreliability or even dishonesty”. It also states that addressing someone as “boy” or “girl” “may cause offence”. Instead, officers and firemen are instructed to use the phrase “young people”.

The same guide also warns against the phrases “manning the phones”, “layman’s terms” and “the tax man”, for “making women invisible”.

The Metropolitan Police warns its staff about “common errors” to watch out for in their language. It says “homosexual” should be avoided and “gay” used, but that “straight” should not be used and “heterosexual” should. “Homosexual” should only be used in connection with legislation, according to the force.

London Fire Brigade instructs its staff not to use the terms “businessmen” or “housewives”, because it says they “reinforce outdated stereotypes”.

For the same reason, it tells workers not to call themselves “firemen” – they are “firefighters”. Other organisations have discouraged using the terms “postmen” and “binmen”.

Marie Clair, spokeswoman for the Plain English Campaign, said: “I have never heard of anyone being confused as to what part of the day it is. When the police need absolute accuracy over when something happened, then I am sure they use the exact time. There comes a point when common sense must prevail.”

She also criticised the decision to avoid phrases like “child” and “youth”. “Do you call a two-year-old a young person? Surely we can get greater accuracy in the language we already use, which is non-offensive,” she added.