BT has hit out at the proposed merger of Vodafone and Three in the UK.

According to the UK telco, the deal would "create a merged entity with a disproportionate share of capacity and spectrum," which it notes is unprecedented in the UK and Western European mobile markets.

The telco also warns that the deal "will substantially lessen competition and deter investment.”

BT submitted a response to the UK's Competition and Markets Authority, which had invited parties to provide submissions commenting on possible remedies for the merger.

The report states that a merger would mean that Vodafone and Three own a 61 percent share of the UK mobile network capacity, an unprecedented amount in UK and Western Europe mobile markets.

BT said it agrees with CMA's initial Phase 1 investigation into the merger, with the regulator stating in March that it expects the deal will lead to mobile customers facing higher prices and reduced service quality.

The CMA has since launched a more in-depth probe into the merger, which will run until September 18, 2024. Once complete, it will give a decision on the deal.

Both Vodafone and Three have previously welcomed a more in-depth probe into the proposed union.

Announced in July last year, the £15 billion ($18.5bn) merger, is set to create the UK's biggest telco with more than 27 million customers.

The merger will give Vodafone a 51 percent majority stake in the combined entity, currently labeled as "MergeCo," with CK Hutchison's Three holding the remaining 49 percent.

Last month, the Secretary of State approved the proposed merger, subject to certain conditions, including both companies setting up a "National Security Committee" to oversee sensitive work that could affect national security.

It's unclear, however, if a change of government will bring about a differing position. The UK is set to elect a new government next month, with the Labour Party expected to win comfortably and be in power for the first time since 2010.