working visa & references from previous employers

my new company uses an agency, and they will help apply working visa for me and they ask for several documents. One of those is reference from previous employers. Is it common? and is it nessessary? Most of times, you left a company then it's hard to go back for reference.

1) Is it common, yes. In industries where reputation is important, such as finance, background verification is almost always required. If you need to be registered then it is required by law.

2) Is it necessary, yes.

3) If the company has already given you the contract, it does not have to be a good reference. Just a verification from management or HR that you have indeed worked there and performed duties matching your resume would be enough. Just give them the name, address and phone number of the previous company, and they would do the rest. If your departure caused some grief it is understandable and should not be a problem. If you have made material mis-statements, then it could be grounds for the new employer to rescind the offer. If this is a reference check prior to offer, then you'd better have someone who would speak well of you at the previous company.