HOUSEHOLDERS have been urged not to pour fat or oil down their kitchen sinks after cooking Christmas meals amid warnings over giant "fatbergs" that clog sewers.

HOUSEHOLDERS have been urged not to pour fat or oil down their kitchen sinks after cooking Christmas meals amid warnings over giant "fatbergs" that clog sewers.

Scottish Water says the congealed fat blocks sewers and can damage pumping stations. Fatbergs are caused when the liquid fat cools and hardens into solid lumps.

The agency has teamed up with supermarket chain Waitrose to offer customer free fat traps when they collect their pre-ordered Christmas turkeys. The device catches fat, oil and grease and allows householders to discard it into the bin instead of down the plughole.

The largest fatberg in Britain, said to be the size of a bus, was discovered in a sewer in London.

They are a problem throughout the year but become a particular concern during the festive period when people in indulge in high-fat dishes and cooking much larger meals than usual.

Across the UK, blocked drains rise by 25 per cent during the festive season, with more than 1,000 tonnes of leftover cooking fat poured down sinks - the equivalent weight of two million Christmas puddings.

Scottish Water have dealt with more than 40,000 blockages in Scotland's networks of drains and sewers last year, at a cost of more than £7 million.

It estimated that 80 per cent of incidents were caused by people putting the wrong things down their sinks and toilets.

Paul Severin, of Scottish Water, said: "Once the congealed fat gets into the main sewers, it builds up along with wipes and other items and can cause serious blockages.

"If the fat then gets into the pumps and the pumps fail, we can end up with a serious environmental issue with discharges into rivers or the sea."

The official advice is to leave fat, oil and grease to cool, scrape it into a sealable container and put it in the bin.

The secure fat trap boxes Waitrose is handing out this year to customers can be used to collect and secure leftover cooking fat and oil, so it can be put in the bin, rather than down the drain. The supermarket is giving away 120,000 fat traps to customers in Edinburgh, Glasgow, Newton Mearns, Helensburgh and Stirling.

Quentin Clark, Head of Sustainability at Waitrose, said: ??Fat in the sewers is a big problem, particularly at this time of year. What we are giving away is just a small box, but we hope it will make a big difference. We are committed to supporting local communities and helping to reduce this problem across Scotland and the wider UK.

??By encouraging our customers to use our free fat-traps, we can dramatically cut the percentage of damage caused to drains by fat, oil and grease.??

Conservationists have also warned that fat left over from cooking Christmas meals should not be used to feed garden birds.

Chris Bailey from RSPB Scotland said: "The fat can actually have quite high salt content, which can be bad for the birds themselves.

"Secondly, the fat tends to be quite sticky and while it may provide good food, it can get onto the birds' feathers, which reduces insulation and also waterproofing."

The RSPB said it is better to use other foods, including boiled rice, cake crumbs, bread and bruised fruit to feed the birds.

Professional cooks stressed that leftover fat could also be recycled for use in the kitchen.

Edinburgh New Town Cookery School Managing Director Fiona Burrell said: "Certainly for vegetable oil, which you may have been using to roast potatoes, you can strain that through a coffee filter or kitchen paper and put it into a bottle that you keep for used oil.

"If it is animal fat, like beef dripping, I would put that into a little pot, keep it in the fridge, and then use it again if you are roasting potatoes or other vegetables."