The specially-trained dogs can smell the ink on bank notes

Thirty spaniels, retrievers and pointers trained to sniff out the ink on currency notes will be used on trains and platforms to intercept couriers bringing criminals' cash from Liverpool, Manchester and Glasgow.

Operation Payback is part of a joint four-year operation between Customs and Excise, the Metropolitan Police and the City of London Police.

They say criminals are bringing large sums of money into London to stop it being traced and to fund more crime.

Paul Evans, from HM Customs and Excise, told BBC London: "If you think about the smell of bank notes when they come out of the cash machine, that's what the dogs are able to smell.

"They can smell it from up to 10 or 15 yards away and through clothing and baggage."

The move follows changes to the law last December which strengthened customs officers' powers to seize suspected criminal cash within UK borders.

It follows several days of seizures by police and customs officials in London.

Gold bars worth �150,000 are among confiscated items on display

Bonds worth 2m Euros (�1.38m) were confiscated at a London airport, officers seized 34,000 Euros (�23,500) from a bureau de change in central London and more than �600,000 cash was recovered using a confiscation order.

A �300,000 Ferrari and gold bars worth �150,000 were among other confiscated items on display at Scotland Yard.

Customs Minister John Healey said �1m a week was being seized across the UK - two thirds of which could not have been confiscated before the Proceeds of Crime Act came into force.

He said: "Cash makes the criminal system work. Without cash the criminals cannot fund their lavish lifestyles.

"All of the cash that we seize and take out of the system will be criminal cash that could have been used for drugs crime, gun crime or tax fraud.