Joanna, I forwarded your question and photo to my brother-in-law René, who's a structural engineer. This is what he says:
Very difficult to know how serious this is without understanding more about the global arrangement of what this beam is supporting.
However, the photograph seems to me to show a possible extension of the crack running towards the right of the picture which has been painted over and so this could be historic damage re-appearing or extending. The deeper the crack goes into the depth of the beam the more serious it is . So, on the face of it not too serious in its current state.
Total replacement of the beam is a bit heavy handed but some repairs would be advisable. It might be possible to inject an epoxy based glue into the crack and then clamp the timber together with a good number of wood screws, countersunk for filling and redecorating.
I hope that's a bit reassuring!