Work right in UK with Tier-2 extension application under process, Urgent!

[DocMalik_12]

Dear all,

I am currently working in a UK University on a Skilled Worker (Tier-2) visa. My visa is supposed to expire on 12th December, 2021. My employer is willing to give me a new CoS and I meet all the requirements for visa extension. However, I am not sure what happens if my current visa expires while my new visa application is under process and if I can I still keep working with my employer?
My employer has told me that they would do an ECS check after I submit my new application. I am not sure how ECS works and does it guarantee a positive response if new application has been made before the previous visa expired.

There is an option to make the new application using Priority serivce that takes 1 week for visa processing but it costs £500 extra that I do not want to spend unless really required.

I would really appreciate any support. Thanks.