British Airways and Spain's Iberia are in talks to forge the latest in a series of European airline mergers that could also form the basis of a three-way transatlantic tie-up.
The UK carrier said on Tuesday the talks were about a potential all-share merger, sending shares in both carriers higher.
It said in a statement the discussions had the support of both companies, though it expected it would take several months before terms could be agreed.
Iberia's 23 percent shareholder Caja Madrid is supportive of the tie-up, a source close to the deal added.
The duo are also in talks with U.S. carrier American Airlines about a transatlantic agreement.
BA's shares were up 6 percent at 248.5 pence, valuing the company at 2.9 billion pounds ($5.76 billion).
Shares in Iberia, the smaller carrier, worth about 1.5 billion euros ($2.36 billion), rallied 20.7 percent.
A fresh wave of consolidation has been expected among European airlines seeking to fight the twin threats of soaring fuel costs and falling consumer demand.
Air France and Dutch carrier KLM became Europe's biggest carrier by merging earlier this decade.
BA Chief Executive Willie Walsh said the move made sense in current market conditions.
"The aviation landscape is changing, and airline consolidation is long overdue," he said in a statement.
"The combined balance sheet, anticipated synergies and network fit between the airlines make a merger an attractive proposition, particularly in the current economic environment," he added.
BA was part of a consortium led by private equity group TPG that withdrew a 3.4 billion euro bid for Iberia last November.
TPG declined to comment on Tuesday.
"From an industrial point of view the deal makes the same sense as it did a year ago, but there are still a lot of details that need to be worked out," said BPI analyst Joaquin Garcia-Romanillos.
"Over the medium term they should reap significant rewards in terms of both synergies ... and also in terms of maximizing revenues," added Neil Glynn, an analyst at NCB.
BA has been a shareholder of Iberia for nearly 10 years and currently owns 13.15 percent of the Spanish carrier, while Iberia has taken a 2.99 percent direct stake in BA, on top of exposure to a further 6.99 percent through contracts for differences linked to the BA share price.
BA said both parties were confident of securing regulatory approval, adding that the European Union had already allowed the duo to cooperate widely.
However, a spokesman for BA rival Virgin Atlantic said the deal would work out badly for passengers.
"This potential merger will only fuel BA's dominance at Heathrow ... We all know that dominant players offer less choice and push up ticket prices," he said.
BA and Iberia would control nearly 45 percent of take-off and landing slots at Heathrow, the core hub for flights between Europe and the United States.
A spokesperson for Air France/KLM, Europe's biggest airline, declined to comment.