"We have decided to exercise the corresponding options," he told a teleconference following the release of Lufthansa quarterly results.
Lufthansa currently holds 30 percent percent minus one share of BMI and has the possibility of acquiring - between December 2008 and June 2009 - the 50 percent plus one share stake held by BMI chief Michael Bishop.
BMI appeals to Lufthansa because it controls several take-off and landing slots at London's Heathrow Airport, a key player in air traffic between Europe and the United States.
BMI's other principal shareholder, the Scandinavian carrier SAS, has a 20 percent stake, which it wants to sell.
Lufthansa shares in mid-day trade here were up 4.20 percent at €18.10 ($28.22) on a generally stronger market after the group posted a huge rise in operating
profit owing to the successful integration of its subsidiary Swiss.
"Their key number, operating income, was better than our estimate," said Citigroup analyst Andrew Light.
Late on Thursday, Lufthansa reported that operating earnings in the period from January to March had grown to €188 million ($295 million) from €36 million in the same period last year. Analysts had expected operating profit around €98 million.
"We had an excellent take-off into 2008," a statement quoted Lufthansa chairman Wolfgang Mayrhuber as saying.
"We are on the right course, we are standing out from the crowd," he added.
Lufthansa said that a "selective expansion of the route network and the successful integration of Swiss" contributed significantly to the results.
Lufthansa took Swiss under its wing in March 2005 to expand passenger traffic business amid consolidation of the European air travel sector.
Mayruber said at the time that the deal would generate one-off costs of €101 million but was also expected to generate annual synergies of €165 million from 2008.