LISBON, May 15 (Reuters) - Portugal's largest listed bank, Millennium bcp, said on Wednesday its first-quarter net profit rose 8.4%, driven by robust income at home, despite high costs related to the forex mortgage portfolio at its Polish subsidiary.

It posted a net consolidated profit of 234.3 million euros ($254 million) in the quarter, above an average forecast of 214 million euros in an LSEG poll, as net income at its domestic business rose by 18.4% from a year ago to 203.5 million euros.

Poland's Bank Millennium, half-owned by Millennium bcp, on Friday reported a 49% drop in quarterly net profit to 128 million zlotys ($32.49 million) as costs related to the FX-mortgage portfolio were 640 million zlotys ($162.43 million) after tax.

Chief Executive Miguel Maya told a news conference "it was another good quarter" and the bank achieved a return on equity of 15%, compared with an average of 1.1% over the last 10 years.

He said the bank, which has 60% of its shares trading freely in the market as the rest are parked with qualified shareholders, "now has all the conditions to have a greater free float like most European banks have."

"This is the natural path for a bank as solid as Millennium bcp today," he said, adding that several European banks have a free float of more than 90%. Free float refers to the shares of a company that can be publicly traded and are not restricted.

One route to it would be a potential sale of a major stake by an investor. Sources told Reuters last month that China's Fosun International is open to selling its remaining stake of around 20% in Millennium bcp after disposing of some shares earlier this year.

Consolidated net interest income, a measure of earnings on loans minus deposit costs, rose 4.8% to 696.2 million euros, in line with the forecast.

Its cost-to-income ratio increased to 35% in March from 31% a year ago, mainly due to increases in wages and other administrative expenses in Poland, it said.

The bank reduced total nonperforming exposures by 15% to 1.09 billion euros in Portugal. ($1 = 3.9402 zlotys) ($1 = 0.9211 euros) (Reporting by Sergio Goncalves in Lisbon Editing by Andrei Khalip and Matthew Lewis)