First, check the water is really coming from the boiler. Then check the pressure gauge: high pressure is a common cause you can deal with yourself. Anything inside the boiler is a job for a Gas Safe engineer.
Signs of a leak
Baxi lists these signs: high boiler pressure, an error code, damp surfaces on or around the boiler, dripping underneath it, and corrosion on the pipes.
What you can safely do
Look carefully at where the water is coming from: the boiler itself, or nearby pipes and radiator valves.
Check the pressure gauge. Ideal says a reading well above normal, around 2 bar or more, could be causing the leak.
If the filling loop under the boiler has been left open, close it. Ideal says that's usually why pressure is high.
If the pressure is high, bleed a radiator to bring it down, as Ideal and Baxi advise.
Put a towel or container under the drip to protect the floor.
Call a Gas Safe engineer if
The pressure is normal but the boiler is still leaking. Ideal says that points to wear in the pipework, joints, seals, pump or heat exchanger.
The pressure keeps rising, which can mean a faulty expansion vessel.
Water is near any electrical parts. Don't touch the inside of the boiler.
Ideal warns that gas, water and electricity are a lethal combination in the wrong hands. Don't open the boiler casing or try to repair it yourself. Get a Gas Safe registered engineer.
Why is my boiler leaking water?
Ideal lists high system pressure, a filling loop left open, a faulty expansion vessel, and wear in pipework, joints, seals, the pump or the heat exchanger.
Can high pressure make a boiler leak?
Yes. Ideal says pressure well above the normal range, around 2 bar or more, can be the cause. A filling loop left open is a common reason for high pressure.