Eastwood said: "The CMA should firmly resist any fast track decision on BT's potential acquisition of EE.
"The decision has the potential to radically change the telecoms landscape in the UK - and we need to make sure that customers do not end up getting a raw deal. There's a real risk that customers might end up having to pay substantially more for substantially worse service. This would be a double whammy, so to speak.
"We have a reputation in Europe for having one of the most competitive and open telecoms markets. Let's not defenestrate that for the sake of a little less consideration time. That would be a travesty. It really would.
"There is obviously the risk that consolidation will hurt challenger companies like ours that seek to provide customers with more choice. An enlarged BT group would make getting yourself heard increasingly challenging.
"These are difficult questions, and we think they are deserving of a serious investigation.
"We are proud of the record we have in the market of providing low-cost simple services to our customers with top-class UK-based customer service. We like to think that we have helped to keep BT on their toes. We want to continue to do so."