The final phase of a ban on traditional light bulbs which has now come into effect means retailers are no longer able to order new stock.
Consumers will soon be unable to buy 40W or 25W bulbs in what is the last stage of a phased programme which began in 2009 and has already seen 100W and 60W bulbs banned.
The EU directive is part of a wider effort to decrease the energy consumption of products. Incandescent bulbs are being targeted because they waste 95% of energy used as heat.
Defra said the technology "has not changed for over 100 years and there are much more efficient alternatives available".
Some people are expected to avoid the ban by seeking out "rough service" lightbulbs which are similar to traditional bulbs but designed to be more robust for industrial use. The rough service bulbs are legal because of a loophole in the EU legislation which only refers to "household lamps".
A number of retailers, particularly hardware stores, are selling rough service bulbs online. But the National Measurement Office, the Government agency responsible for enforcing the ban on traditional bulbs, warned householders against buying the rough service versions.
An official guidance paper says such bulbs are "declared by the manufacturer as unsuitable" for illuminating household rooms and adds: "Consideration should be given to the terms and conditions of any household insurance policy if such lamps are used for illuminating your house."
A Defra spokesman said: "The final stage of the EU phase-out of incandescent light bulbs will mean that from September 1, 25W and 40W bulbs will no longer be allowed into the market. However, this does not prevent retailers selling existing bulbs they already have in stock."
DIY chain B&Q said it supports the shift away from traditional bulbs.
A spokesman said: "These days there's an excellent range of energy-saving light bulbs on offer, saving homeowners money and energy, so there really is no need for UK homeowners to use old-style light bulbs. Modern energy-saving bulbs are now much more stylish and highly cost-effective across their whole lifetime. The next generation of energy-saving light bulbs are LEDs which use even less electricity, last longer and reach full brightness instantly."