馬上生效 的英文怎麼說?

Vendor: Hi, I'd like to let you know we've decided to stop supplying you with the materials effective immediately.
Customer: You cannot do that. Our contract says you need to give me at least one month's notice.
Vendor: I did send you an email about it one month ago.
Customer: Our contract says you need to notify me by phone. So that's still a breach. You need to reimburse me for all of the loss due to your lack of supply.