Trading Standards teams warned more than 110,000 suspect cigarettes seized during raids on four Northampton shops could contain rat faeces, lead or arsenic.
Around 6,000 packets and kilos of tobacco were grabbed during two multi-agency operations in the town.
A spokesman for one of the agencies involved said: "The sale of illicit tobacco products hits taxpayers, as duty is not paid.
"Some counterfeit tobacco products have also been found to contain rat faeces, sawdust, lead, arsenic and even dust swept up from floors in the factories where they are produced."
Last week's swoop on two premises near the town centre landed in excess of 2,300 packets of cigarettes — and 7kg of illegal hand-rolling tobacco.
The operation took place as part of Operation CeCe, a National Trading Standards initiative in partnership with HMRC to tackle illegal tobacco.
That followed a similar raid in September that led to the seizure of more than 3,600 packets of illegal cigarettes from two more shops in the town.
Tobacco in the UK must be sold in plain packaging, but Trading Standards are seeing counterfeit plain packaged tobacco entering the market, as well as more colourful packaging intended for non-UK markets.
The operation was led by HMRC and also involved West Northamptonshire Council Trading Standards, Northamptonshire Police and the Home Office.
West Northamptonshire Council cabinet member Councillor David Smith said: “The import and sale of illegal tobacco causes serious harm in our communities and is certainly not the victimless crime that some people seem to think.
“This latest operation underlines our ongoing commitment to tackling this issue and should serve to reinforce our message that we will continue to take action against those who deal in illegal tobacco."
Wendy Martin, the Director of National Trading Standards, added: “The trade in illegal tobacco harms local communities and affects honest businesses operating within the law.
"The National Trading Standards initiative in partnership with HRMC is playing a significant role in disrupting this illicit trade and is helping to take illegal tobacco products off the streets."
If you suspect anyone is selling illegal tobacco in Northamptonshire, contact the Citizens Advice Consumer Helpline on 0808 223 1133.