If you bought it from a dealer, take it back to them and have them look at it. Could be a hole in your fuel fill hose (where you pump your gas into) or you could have a hole in your tank, breather tube could have come loose, almost anything.

After it stops leaking, do you have a certain amount of gas left? Like 1/2 a tank, 3/4, etc. Could be a loose gasket at the place where the fuel pump is located. Again, almost anything. The best thing to do is either take it back where you got it from (if a dealer) and have them look at it (since they should have been aware of something as dangerous as that), or jack it up and try to find out where it's coming from(front, rear, side, top, bottom, etc).

 

Did you buy it from a dealer?

What year, what size engine and what mileage?

 

The 2005 models had a gas tank recall due to potential leaks in the fuel filler area. The recall is performed only when the owner complains to Jaguar about leaking gas, usually when the car is being refueled.

Our car's gas tank was replaced under this recall prior to us purchasing it. I found this information searching the car's history prior to making my offer to buy it.

Don't know if this particular recall applies to your 2003 model or not. You may want to call Jaguar Corporate and ask....

 

The problem might be the connections on top of the tank. One guy took it apart and found the problem right away. The good thing is you can access most all of it from the top. Remove the back seat lower cushion and there are two access ports to service the two fuel pumps and other hardware.

Here is a picture of the Lincoln LS fuel tank which is very close. Also a picture of the access ports under the rear seat.
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