Ford ka shakes and wont accelerate

Hi, I have a ford Ka3 and when I drive the car shakes and wont accelerate when I get to around 40mph. If I pull over or slow to around 15mph the shaking stops and I can drive for a few more miles but then it starts shaking again, I have to drive about 200 miles tomorrow but I am now a bit worried about how dangerous this is. Any ideas on what it might be?