Thames Water will spend �5m next year clearing the sewage system

The company says every year it flushes 1,000 tonnes of hard, congealed fat through its sewerage system.

The firm said any one of the 60,000 blockages caused by this clogging can force raw sewage out of a manhole and flooding into homes and gardens.

It advises fat to be poured into tins, then put it in a plastic bag, before being thrown in a bin.

It is also pioneering a scheme in Bromley, south-east London, where used cooking oils are collected from local restaurants and caterers.

Hard as concrete

It is then turned into the eco-friendly 'biodiesel' that is being used in the fuel tanks of local authority vehicles.

The firm will spend �5m next year clearing the sewage system and said it will prosecute food outlets that pour too much grease down the drain.

Liquid fat, once underground, cools, hardens and can form a plug as hard as concrete.

In 2003, Thames Water said solidified fat, oil and grease blocked about a kilometre of pipe between Westminster and Piccadilly in central London.

It took a team of 10 men three months to remove it by hand, costing about �250,000.