UK shoppers may be able to buy alcohol and cigarettes from Europe over the internet at bargain prices if the EU Court of Justice backs the move.
The case hinges on wine bought in France by a Dutch shopper
The court will decide this month if goods from EU states can be delivered to them in Britain free of UK duty.
An adviser to the court has already backed the change, which could mean reduced tax income for the UK Treasury.
But the Treasury, which wants the court to reject the earlier ruling, says the way it interprets duty laws is correct.
An advocate-general, Francis Jacob, gave his independent legal advice and opinion to the EU court last December.
This is usually a good indication of the court's eventual decision, although it is not obliged to follow the advice. The judges will make their decision on 23 November.
If it follows its legal adviser it means shoppers will be able to order alcohol and cigarettes from overseas using their keyboard or phone.
There are large potential savings to be made from shopping for alcohol on mainland Europe, where there are lower alcohol and tobacco duty rates in many countries.
The big UK supermarkets have tried to cater for the British customer abroad - opening shops aimed specifically at the "booze cruise" market.
And the Wine and Spirits Trade Association said UK based retailers may "struggle to compete" on wine sales against nations such as France, where duty was minimal.
They could suffer under the new rules - but the Treasury could suffer more through loss of revenue.
The case comes down to where excise duty on alcohol and cigarettes should be charged. In general, it is chargeable in the member state of final destination.
However, a provision allows for excise duty on products "acquired by private individuals for their own use and transported by them" to be charged in the member state of purchase.
In the present case a private individual, resident in the Netherlands, purchased duty-paid wine in France for his own use. He did not transport the wine himself, but engaged a transport company to do so.
The Dutch tax authorities now wish to levy excise duty on the wine, and the UK government is supporting them in its action.
"We are confident that our interpretation of the law is the correct one," said a Treasury spokesman.
"Obviously we are watching the outcome of this case with great interest along with all the other interested parties.
"However, it is very important to remember that the advocate-general's opinion is just that - an opinion."