Dear Jacek,  I am sure it is not Linum narbonense, which has large leaves and grows in brachish marshes. This plant is similar to Linum bienne, but I am not sure

Best wishes

Dear Daniele thank you. I am also considering L. suffruticosum or L. tenuifolium. What's your opinion? By the way, my plant is glabrous so it's not L. hirsutum.

Dear Jacek, I don't know Linum suffruticosum; Linum tenuifolium is similar to Linum bienne, but generally smaller and with pink/pale blue petals. It lives on limestone. You could look at some beautiful photos with also particular plant traits on www.actaplantarum.org

Thank you for this remark and your identification. I used the pictorial key (in German language, link below) and obtained L. bienne. Thank you very much. Best regards,

Jacel

http://www.mittelmeerflora.de/Zweikeim/Linacea/linum.htm

Use this key from Flora croatica. I can not see whether the sepals are glandular (L. tenuifolium) or not. L. hologynum, similar to L. bienne (syn. L. angustifolium), is missing in the key

Dear Michal,

I have finally used this key. Sepal margins are glandular, leaf margin is minutely scabrous. I got Linum tenuifolium L. Thank you a lot for posting this key. Best regards from Poland,

J a c e k