Former West German chancellor Willy Brandt celebrated his 65th birthday in a hospital bed where he is recuperating from a "mild heart attack" he suffered last month.

Although Brandt, top government officials and the Social Democratic Party have publicly brushed aside speculation that Brandt will nto acknowledge that there is deep concern about whether his leadership days are over.

Brandt, who resigned as chancellor in 1974 following relevation of a spy scandal, has remained chairman of the ruling Social Democrats as well as becomignpresident of Socialist International and head of the north-south commission dealing with problems of the developing world. He is slated to run for election to the new European Parliament.

Offically, the party says Brandt will take a two or three-month recuperative leave and by the middle of 1979 will be working half-time but his friends think it may be a year before be can bounce back.

Brandt, more interested these days in international affairs, has tended to stray occasionally from party chairmanship duties in the past. Whenever he has done so, it has hurt party unity and election performance.

Brandt's forced absence for a year or so could have similar consequences and party sources acknowledge that the loss of Brandt-internationall the best known West German politician-could be very serious.