It's hard to tell from the photo ( is that large vertical streak said crack? ), but if your frame is cracked, don't use it.
Riding on a cracked frame is risky as it could result in a catastrophic failure / injury / death.
It looks like that's probably a steel frame, so find a local frame builder ( not your buddy who's handy with a welder ), and depending on the damage they may be able to repair it for you.
bicycles.stackexchange.com/users/5237/Scott Hillson
I am going to get beat up for this but
steel frames don't fail catastrophically.
Pull that post.
If the end of the post is in the middle of the crack then look for a longer post.
Get a seat post that extends at least 2" below the bottom of the crack and ideally 4".
The post reinforces the frame.
Mark the two ends of the crack.
If the crack grows stop riding.
I know this is kind of ghetto but put a couple hose clamps on the crack.
That old frame is not really worth a professional repair.
But you may be able to get a couple more years out of it.
If that was an aluminum frame I would tell you to throw it away.
bicycles.stackexchange.com/users/7785/paparazzo
Honestly, how much did this bike cost?
It looks to be at least 30 years old, judging by the tire and brakes we see in this picture, and even when it was new, I doubt it was worth repairing this kind of damage.
The cost of repairing it would be far in excess of buying a new bike.
Now, if you'd spent $1500 on it, even 30 years ago, well, maybe it's worth it.
But probably not.
My guess is that $300 would have been an excessive amount when it was new, and the bill to repair this crack would be larger than that.
bicycles.stackexchange.com/users/2442/Ernie
Dawes Galaxy?
Looks like an older model - so a good quality touring frameset - possibly Reynolds tubing.
Anyway, what price do you put on sentimentality?
I'm not sure where the idea of long seatpost comes from as the picture doesn't show it.
But if you are UK based a company like Argos cycles can easily fix that frame for you.
Its a welding job - so the frame could be resprayed and Dawes decals reapplied.
On their website they quote £120 for a seat tube repair
but this is a complete replacement of the seat tube.
So yours may be different.
A full respray is £135.
bicycles.stackexchange.com/users/16979/OraNob
A couple of years ago we had a new seat tube fitted to my wifes bike (a 20 year old Claud Butler Lady Dale) after it sheared where it joined the top tube - this was on a step through frame.
Fortunately we live close enough to Roberts, one of the few frame builders in the UK, to be able to deliver it.
I had to strip the frame for them and it cost about £150.
They completely stripped the paint off the frame
so we had it powder coated - approx
£100.
We did this as she really likes the bike and finds it comfortable for long rides.
I don't think that we could get something similar for the price.
If you want to get a new seat tube, you will probably have to send it away.
None of the local bike shops would consider it - they make money selling new bikes after all.
I think that only Mercian and Roberts offered a quote.
Hopefully this has given you an idea of the costs and what is involved.
If you like your bike as it is then it can be a viable option.
I suspect that the longer seat post may be the cheaper alternative.
bicycles.stackexchange.com/users/4447/DanS
I just fixed mine
, I took it to a local welding shop in London, they charged me only £8, it's not pretty
but it works fine.
I made this little video
bicycles.stackexchange.com/users/35337/Sabba Keynejad
I noticed the other day that my frame has a crack in it! I believe that it's my seat post causing the problem. A couple of questions really, a) how serious is this? and b) can it be repaired?   Edit: I've added a picture of the height of the seat-post to give some extra context on whether or not the frame is too small for me. I've never thought that the seat was crazy high or anything; is it?
What can I do about a cracked frame?
bicycles.stackexchange.com
<frames><steel><crack>
bicycles.stackexchange.com/questions/28854
bicycles.stackexchange.com/users/1560/Matthew
Riding on a cracked frame is risky as it could result in a catastrophic failure / injury / death.  Some people believe it is worth repairing a frame crack but others think that the cost of repairing it would be the same or more than buying a new bike. All are agreed that riding with a cracked frame is dangerous.  Generally you should never use any bike with a cracked frame, but this may not actually be the case here. Unlike carbon or aluminium, steel frames do not fail catastrophically and can be welded and resprayed. If the crack grows, stop riding. Some users argue that the necessary professional welding would be more expensive than the bike was worth, and suggest that the bike should be replaced or the cracked frame strengthened with a longer seat post or hose clamps.
Generally you should never use any bike with a cracked frame, but this may not actually be the case here. Unlike carbon or aluminium, steel frames do not fail catastrophically and can be welded and resprayed. If the crack grows, stop riding. Some users argue that the necessary professional welding would be more expensive than the bike was worth, and suggest that the bike should be replaced or the cracked frame strengthened with a longer seat post or hose clamps.