IAG also announced its intention to buy bmi, a Lufthansa division, in an attempt to boost profits at its Heathrow hub, the airline said on Friday.
British Airways and Iberia parent said it had reached an agreement in principle with bmi's German owner for the sale of the loss-making unit - the second-largest carrier at Heathrow - with any deal subject to due diligence and regulatory clearances.
"We're confident we can make a success of bmi and will look to expand our network, particularly our long-haul network through this deal," IAG's chief executive Willie Walsh told reporters, adding that IAG did not yet have exclusivity on any deal but believed its offer was more attractive than others Lufthansa had received.
Rival UK carrier Virgin Atlantic said it had made a bid for bmi and was still "working with Lufthansa".
IAG said it expects the purchase agreement to be signed in the coming weeks and for a transaction to be completed in the first quarter of 2012 with analysts put a £300 million price tag on the deal.
With 9 percent of the take-off and landing slots, bmi is the second-largest carrier at Heathrow, Europe's busiest airport. A deal offers IAG the opportunity to grow at Heathrow, which is operating at full capacity after plans to build a third runway were scrapped.
IAG shares were down 4 percent at 161.1 pence by 8:35 a.m. British time, valuing the business at around £2.1 billion.
IAG's BA is the largest carrier at Heathrow with a 43.1 percent share of the slots - ahead of Virgin in fifth with 3.1 percent - and has most to gain if it can snap up bmi slots.
Walsh said he was confident the bmi deal would be cleared by regulators because IAG's holding at Heathrow is small compared with rivals at other European hubs - Lufthansa holds two-thirds of the slots at Frankfurt, while Air France-KLM has 59 percent at Charles de Gaulle in
Paris and 57 percent at Amsterdam's Schiphol.
IAG, Europe's second-biggest airline group by value behind Lufthansa, said operating profit in the three months to the end of September fell to €363 million (£311 million) from last year's third-quarter profit of €528 million.
by a quarter to €1.39 billion in the period and weak demand at Spain's Iberia offset strong trade at British Airways.
"Rising fuel costs are the biggest challenge to the industry in the short-term along with potentially weaker demand in 2012," said Walsh.
The company had been expected to report a third-quarter operating profit of €350 million, according to the consensus analyst forecast supplied by IAG.