informer:
(plural: ‘informers‘; pronunciation: the letter ‘i’ rhymes with the letter ‘i’ in “ink” — for the right pronunciation of this word, please click here)

a police informer (R) passing on information
a person who informs (to give information) against another or on matters of importance to higher officials, especially one criminal against another to the police {Note: in this context, the words ‘informant’ and ‘informer’ are interchangeable, i.e. either word can be used.};
a person who gives out (reveals) somebody’s or some organisation’s secret information to others for money;
an informant ( ← ← previous entry)

cartoon showing an informer (whistle blower) giving information about a person
We were all dumb-founded when we heard the news that our very cooperative colleague was an informer set up by our manager to know what the employees were doing and how they were doing!
You’d be surprised to know that most of the police informers are petty thieves who make their living doing minor crimes, such as selling contraband goods, pickpocketing, etc.
Police informers play a vital role in solving most of the street crime; however, when it comes to organised crimes, nobody likes to act as informers for fear of their own life.
In countries where banned political and religious groups operate their activities underground, the security forces make their moves to apprehend them with the information they get from their well-paid informers.
