We know traces of horsemeat were found in some frozen food. But how did it get there, what is safe to eat - and who had the best "horse-burger" jokes? Channel 4 News has the answers.
Which products were found to contain horsemeat?
The FSA is investigating two cases of "significant horsemeat" in Tesco's frozen burgers and Findus lasagne.
Findus conducted 18 tests on a range of their beef lasagne products. According to the FSA, the results showed "levels of species content" by bands - and eleven of the results showed horse DNA in the range of 60 per cent to 100 per cent.
Last month, in a sample of 27 beef burger products, a tiny amount of less than 1 per cent of horsemeat was found in the food samples of five frozen burger products from five separate Uk and Ireland-based supermarkets:
- Tesco beef quarter pounders: 0.1 per cent,
- Aldi's Oakhurst beef burgers: 0.3 per cent,
- Lidl's moordale quarter pounders: 0.1 per cent,
- Two types of Iceland's quarter pounders: 0.1 per cent,
- Dunnes Stores' (based in Ireland) flamehouse chargrilled quarter pounders: 0.1 per cent.
But in Tesco everyday value beef burgers, 29 per cent of horse meat was found relative to beef - almost a third of the meat in the burger.
A Tesco spokesperson told Channel 4 News that 26 frozen burger lines have been recalled from the shelves until further notice as a precautionary measure - those which were found to contain horsemeat, and any other burgers produced at the North Yorkshire site that the horse and pig samples were traced back to.
Some 31 beef meal products such as cottage pie, beef curry pie and lasagne were also tested, with 21 found to be positive for pig DNA, but all tested negative for horse meat.
How did it get there?
Findus withdrew the lasagne products after its French supplier, Comigel, raised concerns about the type of meat used in the lasagne. In a statement the company said: "We are confident that we have fully resolved this supply chain issue. Fully compliant beef lasagne will be in stores again soon."
The French company also supplies Aldi and Tesco, which have both withdrawn Comigel lasagne and spaghetti bolognese from their stores as a precautionary measure.
The initial investigation into Tesco burgers by the Food Safety Authority of Ireland (FSAI) traced the horse and pig DNA found in unsuspecting beef products to two processing plants in Ireland - Liffey Meats and Silvercrest Foods - and the North Yorkshire based Dalepak Hambleton plant in the UK.
After dropping Silvercrest, Tesco said the company used meat in its products that did not come from a list of approved suppliers and was from outside the UK and Ireland.
The Food Standards Agency now believes a "filler product" made from horsemeat and beef that was found in the contaminated burgers came from Poland.
When will we know if the products are safe?
The FSA said it has no evidence to suggest that the horsemeat found in Findus products contituted a food safety risk. However, it has ordered Findus to test the lasagne for the veterinary drug phenylbutazone, or "bute". Animals treated with phenylbutazone are not allowed to enter the food chain as it may pose a risk to human health.
The FSA added: "People who have bought any Findus beef lasagne products are advised not to eat them and return them to the shop they bought them from."
Meanwhile, in a preliminary investigation into Dalepak in Yorkshire, the FSA conducted tests onseven samples of burger product and concluded that "neither horse nor pork DNA was detected in any of these samples".
The FSA is expected to focus on whether the plant has suppliers in common with Silvercrest in Ireland.
But this is only the first step: further suppliers and ingredients will also need to be tested, the FSA told Channel 4 News.
Last month the restaurant chain Burger King announced that it would no longer be sourcing burgers from the Irish supplier ABP which owns Silvercrest, even though ABP has insisted that meat for Burger King was stored and processed separately. Burger King said this was a "precaution" which might mean that some products were temporarily unavailable.
What burgers are free from horsemeat?
The burgers that tested negative for traces of horse - and pig - DNA were: Iceland beef burgers, Supervalue daily basics beef burgers and simply M&S British beef burgers.
Many fresh burgers are still on sale in the seven supermarkets that recalled frozen burgers, as are branded burgers that aren't made at the plants implicated, such those from the Birds Eye brand. Otherwise you could always pop to the local butchers, where staff should be able to tell you where the meat is sourced - but you will have to pay more for the privilege.
Is horsemeat harmful to eat?
Far from being dangerous to eat, horse meat is considered a delicacy in some countries, namely our more adventurous foodie neighbour France, as well as Italy and Japan. The FSA was keen to point out that "there is no food safety risk to consumers from these products".
If the products tested had contained horse medicines - for example phenylbutazone, a commonly used medicine in horses - the risks to human consumption would be much higher. This is banned from use on animals in the food chain, but so far it has not been found in the beef samples tested.
What are the best #horsepuns?
Despite the outrage of the Great British Public, the horsemeat burgers gave rise to a herd of horse-related puns. The hashtags #horsepuns and #burgergate soon took hold on Twitter, along with references to "gallop polls" and horse meat being part of a "stable diet".
"Tesco press office sounds slightly manic. Must be hard work putting together a statement on the hoof," tweeted @jimwaterson, while @GeorgeNairn posted: "Tesco's burgers, a mane part of a stable diet."
But the last laugh has to be reserved for Tesco, whose horse-related tweet to its 47,000 followers was apparently scheduled to send before the horsemeat burger revelations: "It's sleepy time so we're off to hit the hay! See you at 8am for more #TescoTweets".