ATAS is the Academic Technology Improvement Scheme. It seems to be a vetting system for students coming from outside the United Kingdom with regard to terrorism.
https://www.atas.fco.gov.uk/public/welcome.aspx
The website does not display properly on my PC, however, from the FAQs:
"One of the UK Government’s strategic international priorities is to 'make the world safer from global terrorism and weapons of mass destruction (WMD)'. The ATAS is a tool the UK government uses to help meet this goal; it is designed to be a light touch scheme which should not inconvenience students.
If you meet the criteria the Immigration Rules state that you need to have an ATAS certificate before you apply for your Visa / EC / EoS. It is important that the details on your ATAS application match the details on the offer letter you have received from your university. You will need this certificate when you apply for your student Visa / EC / EoS. If you apply for a Visa / EC / EoS without obtaining an ATAS certificate, and you are a student who needs one, your Visa / EC / EoS application will be refused. Therefore, you should read this information carefully so that there are no delays with your application."
There's an e-mail address at the bottom of the page and I suggest you e-mail them to find out more about your application (atas@fco.gov.uk). Otherwise you may somehow have to ring our Foreign Office or the British High Commision in Delhi to find out what is happening.
Sorry I can't be of more help, however, I've got to admit this the first I've heard of the scheme.
Ian