There's always the possibility of a fire starting and spreading if escaped fuel is
present, while flammable vapour from petrol can cause an explosion if it's ignited.
The portable or close-coupled fuel tank must be made of a suitable material for
the fuel used and maintained in a sound condition to minimise this risk.
In this context a portable fuel tank is one that can be carried on and off the
boat and is designed to be connected by flexible piping directly to the engine.

A close-coupled fuel tank forms an integral part of the engine.
To control any accidental escape of fuel a shut-off valve must be fitted to the
fuel supply line into the engine. A bayonet connection on a portable fuel
tank/supply system can act as a shut-off valve.

To help prevent any of the components of the fuel supply system from failing
and leaking fuel, the system must not be changed or modified in any way from
that supplied or approved by the component manufacturer.

Examiners will check that the fuel line itself is still pliable and is not showing signs of deterioration, such as cracking, which can be caused through old-age or by being in contact with the wrong fuel. It's always worth checking the condition of the fuel line on a regular maintenance basis to prevent any nasty surprises. [5.1/5.2]

One boat owner had a nasty shock on a hot and sunny day. The petrol stored in containers on his deck was left in direct sunlight on one very hot and sunny day while the owner visited a local attraction. On his return, the boater decided to refuel his small outboard-engined boat, but forgot that vapours from the fuel would build up under pressure, because of the heat affecting the containers. When he opened the lid, petrol vapour escaped under high pressure and was ignited by what is believed to have been a discarded cigarette. The boater suffered significant burns to his arms as a result. [5.3]

To minimise the risk of putting the wrong fuel into the fuel tank, it's advisable to
clearly mark your portable fuel tanks with the type of fuel they contain.

To avoid fuel and fuel vapour leaking into the boat from spare fuel tanks that are
not directly connected to the engine, the tanks must be stored in accordance
with Standards 7.2 through to 7.8, e.g. in a fire-resistant drained locker. This will
help to ensure that any escaping fuel or fuel vapours drain overboard, rather
than into the boat.

Small amounts of fuel can be diluted if they escape overboard. You should
contact the Environment Agency Pollution Hotline on 0800 80 70 60 (24 hrs)
or the Scottish Environment Protection Agency on 0345 73 72 71 to report
instances where large quantities of fuel or other substances escape into a
watercourse. [5.3]