I would think more likely is a faulty ABS module,whatever the problem you should on no accounts be driving the car until it is repaired.

I agree - in fact (dependent on the age), there was a recall for this exact issue. I've just discovered this as I have one in with the same problem.

Recall ref for mine is R/2009/050: HARD BRAKE PEDAL MAY OCCUR DURING ENGINE WARM UP

I too suspect this is loss of vacuum for some reason (collapsing pipe?), would be my first check anyway.

You could try this if you feel confident, it might help diagnose...first of all familiarise yourself with the sound and feel of your brake servo, start with a full head of vacuum then switch engine off.

Then press the brake pedal several times to use the vacuum up, if quiet you should hear the vacuum exhausting each press and feel the brakes gradually harden off as servo assistance is lost, it will show just how poor the brakes are with no vacuum.

Once familiar and confident with this, next time this happens if possible and only if safe to do so switch off the engine immediately and stop the car on the handbrake. Then test for vacuum as above, it should be immediately obvious if vacuum loss is the fault.

Take that as a no then.

Ayrshore's recall is on the AA website:

During engine warm up it is possible that the non return valve within the vacuum pump may stick. It is possible that if sufficient brake applications are made before the valve releases, the vacuum stored within the brake booster will be used up and a hard brake pedal and longer stopping distances may result.