Barclays has confirmed that it is in exclusive talks with Dutch rival ABN Amro about merging to create a bank worth as much as £80bn ($155bn).
Barclays may become a takeover target itself, analysts say
The UK's third biggest bank says that the talks are at an early and exploratory stage and there can be no certainty of a transaction being done.
A Barclays-ABN merger would create a bank with 47 million customers and 220,000 staff worldwide.
It also may prompt other bids for ABN - and for Barclays itself, analysts said.
On Monday, the Dutch firm's shares rose 8% in response to the news, but Barclays shares closed down 0.8% as investors awaited more information about the situation.
In recent years, ABN - the Netherlands' biggest bank - has cut jobs to control costs so it can expand into new markets and face down more competition.
Barclays, the UK's third-largest bank, wants access to ABN's foreign businesses, in particular its Asian operations, analysts said.
Barclays and ABN held talks two years ago when they first explored the possibility of a merger.
"It has been the great dream of Britain's big banks to merge with a large continental rival, and for Barclays that dream may at last be coming true," said the BBC's business editor Robert Peston.
"Simply to have got to the stage where it is in exclusive merger talks with ABN is quite a feat, given the political, regulatory and culture obstacles to such cross-border deals."
Royal Bank of Scotland and Italy's Capitalia are among banking groups who are also thought to be interested in buying some or all of ABN's businesses.
In recent months two UK hedge funds - TCI and Toscafund - have criticised the Dutch bank for its poor performance and both are understood to be calling for a shake-up at the group.