Lottery operator Camelot has been told to make it clearer that Britons abroad in certain countries are barred from buying a ticket.
Countries such as the US and Spain have local laws barring people including British tourists from taking part in foreign lottery draws while they are in the country.
But the National Lottery website can be accessed from anywhere in the world and it has led to claims from some Brits aboard of buying winning tickets, but then being unable to claim any cash.
Stephen McGlade, from consumer group Which?, said Camelot must do more to prevent such situations occurring.
He told the BBC: "What we'd like Camelot to do is to invest more in the website so it makes it clear to people playing the lottery abroad that it is void with tickets if they purchased them abroad.
"Secondly, there should be measures in place so that if I am in Spain and I play the lottery, I'm barred from doing so."
A section of Camelot's website labelled "Important Information" does warn that any ticket bought in a country where playing the lottery online is banned will not be valid.
"We would advise players to assume that it is unlawful to purchase a ticket whilst abroad, and to only buy their tickets whilst located in the UK or Isle of Man," the information says.
But the group said it is currently unable to automatically bar people playing in relevant countries.
Camelot spokesman Rob Dwight told the BBC: "We have looked at a range of software options over the years and have actually tested them out and we will continue to do so.
"Obviously, the technology in this area is constantly evolving.
"What we want to do is make sure that people who are entitled to play can play, and those that shouldn't be playing don't play."
This weekend's lotto draw is a £7 million rollover.
Mr Dwight stressed today that Camelot reminds online players they should only play in a country where it is lawful to do so and advises them only to buy their tickets while in the UK or Isle of Man every time they sign into their accounts or buy their tickets.
"We also do this when they initially set up or purchase their direct debit."
He added that information about overseas play on the website "complements our regular emails and account notifications to players".
He said: "Over the last year we have sent a number of such communications to our players advising them only to purchase lottery tickets while they are based in the UK and Isle of Man - these are issued at certain periods of the year when players are more likely to go abroad."