After BT warned that a £15 billion merger between Vodafone and Three UK would cause prices to skyrocket, the two companies have come out swinging — pledging to cap their lowest-cost mobile plans at £10 for two years.

Vodafone Group Plc and CK Hutchison Holdings Ltd.’s Three told antitrust regulators they’ll keep a £10-per-month plan available for two years if the companies are allowed to merge in the UK.

Vodafone and Hutchison's Three UK will commit to low tariffs on their value brand for two years and give better access to wholesale operators, making new pledges to the regulator to try to get approval for their $19 billion merger.

The firms said they will also encourage so-called virtual providers to access their network, so smaller operators can offer good deals for customers. Furthermore, Vodafone and Thr