Thousands of tonnes of horsemeat probably entered the food chain in the three years before last year's food crisis, according to a frontbench Labour MP.
Mary Creagh, who was shadow environment secretary at the time of the scandal, says she has evidence which proves criminals substituted horsemeat for beef on a massive scale.
The discovery of horsemeat in processed beef products sold by a number of UK supermarket chains last year resulted in a series of product recalls.
"The number of horses slaughtered in Britain has halved over the past year - so what that tells you is probably 50% of the horses that were being slaughtered in 2010, 2011 and 2012 were destined for some sort of criminal meat trade," Ms Creagh told BBC Radio 4's File on 4 programme.
The official government figures show that in 2009, there were 5,100 horses slaughtered. In 2010 there were 8,854, in 2011 there were 9,011 and in 2012 there were 9,405.
That figure, for the 11 months of 2013 following the crisis, fell by more than half to 4,505.
"There are [now] much stricter checks in abattoirs and the paperwork is being checked a lot more carefully. Veterinary inspectors are alive to the potential for horses to have more than one microchip in their neck and it is no longer in the interest of would-be criminals to take the risk of getting caught," said Ms Creagh, now shadow transport secretary.
She added that the number of slaughterhouses approved to kill horses in the UK had also fallen, from seven in January 2013 to a current total of four.
"The anecdotal evidence from animal welfare charities is that there was a huge increase in the number of abandoned horses, there was a huge increase in horse passport fraud and there was a huge increase in horse slaughter."
There was a glut of unwanted horses after the 2008 financial crisis, with people no longer able to afford to keep a horse.
"They were horses that may have been treated with bute [a drug given to horses], had fresh chips put in their heads and new passports written for them - so they were being cleaned... and then taken to slaughter.
"Not all of those horses I think were exported, some of them may have stayed in the UK. I am in no doubt that they were entering the food chain and being passed off as beef," Ms Creagh said.
The National Audit Office has already criticised the government, suggesting it should have picked up on "warning signs" as long ago as 2010.
In its investigation into the crisis published in October 2013 the NAO pointed to economic recession putting pressure on household bills and leaving the way open for food fraudsters to introduce horsemeat into the food chain.
Sian Jones, the audit manager responsible for the report, told File on 4:
"The price of beef on our shelves was staying the same. The price of horsemeat wholesale was going down... horsemeat looks and tastes an awful lot like beef so it's an ideal candidate for adulteration. We felt on reflection that these were signs that intelligence should have picked up on - and didn't."
'Know the loopholes'
In a statement, the Food Standards Agency said:
"[Our] role is to ensure consumers are protected and our focus is on working with the food industry and local authorities to make sure this happens. For example, we are working with our local authority partners to do more authenticity testing and we have increased funding to support them in this work to £2.2m.
"We asked Prof Pat Troop to review the FSA's response to the horsemeat incident and we are working on her recommendations to improve our intelligence gathering, review the way we respond to incidents and look again at our powers."
Environment Secretary Owen Paterson declined File on 4's request for an interview.
A spokesperson for the Department of Environment, Food and Rural Affairs said a review of Britain's foot supply networks by Professor Chris Elliott "recognises that UK consumers have access to some of the safest food in the world" and added: "We want to keep it that way.
"In the last year, we have taken action to prevent food crime, including increasing unannounced inspections of meat cutting plants and boosting funding to £2 million to support local authorities' food sampling programme."
In her first interview as head of the European Union's Food Fraud Network, which was set up in the wake of the horsemeat scandal, Carmen Garau told the BBC the unit is already investigating 40 cases of beef being substituted for horsemeat.
"It has become apparent that this... can happen more and more in future because fraud has become technologically advanced, they know the legislation and they know the loopholes in the legislation.
"They know how to bypass and how to avoid controls in some cases, so we do need to organise ourselves to address that sort of phenomenon."
Ms Garau says the EU Food Fraud Network, which has members from each of the 28 member states plus Iceland, Switzerland and Norway, is currently investigating 20 different types of food fraud.
These include species substitution in fish, water being injected into frozen seafood to increase its weight and the substitution of honey for sugar syrup.
"After the horsemeat crisis, we realised that one of the main problems in enforcing food legislation across Europe and tackling food fraud across Europe was the fact that member states were not using to the full the instruments and the tools for administrative co-operation across borders.
They have to co-operate with each other when the violation of food law is intentional, is cross border, and is economically motivated," she said.
File on 4's investigation into food fraud is on BBC Radio 4 at 20:00 on Tuesday, January 28.