Lufthansa Group is strengthening its portfolio of European carriers by agreeing to acquire a 55% stake in Brussels Airlines it does not own.

The deal is seen as part of a plan to expand its low-cost off-shoot Eurowings through acquisitions, Reuters reported.

Plans are in the pipeline for Lufthansa to take aircraft for Eurowings from Air Berlin to help it become the third largest budget airline in Europe, a position currently held by Norwegian.

Lufthansa said earlier this year it was discussing how to bring Brussels Airlines, which has  a fleet of 49 aircraft into Eurowings, which operates about 90 aircraft. 

Lufthansa bought 45% of Brussels Airlines owner SN Airholding in 2009 for €65 million, with an option to acquire the rest from 2011.

Because Lufthansa has loaned €45 million to Brussels Airlines, the German carrier can buy the rest for as little as €2.6 million more, according to local reports.

A decision about whether to exercise its option to buy all of Brussels Airlines was delayed by Lufthansa after fatal attacks at the airport and metro system in the Belgian capital in March.

Brussels Airlines flew a record 7.5 million passengers in 2015 and made a record net profit of €41.3 million. Numbers decreased in March and April as a result of the attacks but have been recovering since.

The Lufthansa supervisory board said it hoped to finalise the deal early in 2017. The German group already comprises Austrian Airlines, Swiss, Eurowings and Germanwings.