Britain's competition regulator will investigate Nationwide's £2.9 billion acquisition of rival Virgin Money.
The Competition and Markets Authority (CMA) said on Friday morning it had launched a merger investigation into the deal.
Stakeholders were invited to present their opinions on this transaction, and a response deadline of June 14 was set.
It said it was considering whether the deal, the biggest merger of British banks since the financial crisis, could result in a “significant weakening of competition” within the UK market.
The CMA said it would decide by July 26 whether a more thorough stage 1 investigation into the deal was needed.
In March, Nationwide and Virgin Money agreed a deal.
Nationwide has signed a takeover deal with a 220p per share offer for Virgin Money, including a planned 2p per share dividend payment.
Last week, 89% of Virgin Money shareholders voted in favor of the move, helping pave the way for the deal to be completed.
Virgin Money chief executive David Duffy will step down once the deal is completed, expected in the last three months of 2024, and Nationwide boss Debbie Crosbie will lead the enlarged group.
The planned acquisition will bring together the UK's fifth and sixth largest retail lenders, creating a combined group with around 24.5 million customers, more than 25,000 staff and around 700 branches.
However, the move marks the end of the Virgin Money brand and Nationwide plans to rebrand its Virgin Money business to Nationwide within six years, although it will initially retain both brands.