Children's charities have welcomed the jailing of former British pop star Gary Glitter for child abuse in Vietnam.
Gary Glitter denies any wrongdoing
Ecpat said it was pleased justice had been done but "disappointed" he had not received the maximum seven-year term.
World Vision UK called for tighter controls on registered sex offenders such as Glitter, real name Paul Gadd, from travelling abroad.
A Home Office spokeswoman said the UK had one of the world's most advanced systems of managing sex offenders.
Glitter, who shot to fame in the early 1970s with hits such as Leader of the Gang, was jailed for three years by a Vietnamese court for committing "lewd acts" with two young girls.
Christine Beddoe, of End Child Prostitution, Pornography and Trafficking (Ecpat), called on UK police to look at bringing charges against Glitter should he return.
"We would like to see a further investigation by the British authorities to see if it would be possible to bring a prosecution against him here," she said.
A spokeswoman for the NSPCC said a legal loophole on sex tourism abroad needed to be closed.
"Sex offenders must be brought to justice no matter what country they abuse children in," she said.
World Vision UK's Philippa Lei said existing laws should be "used and enforced more strongly" to ensure cases such as Glitter's did not happen again.
A Home Office spokeswoman said the government had given police and courts powers to impose a "stringent conditions" where there is any risk to children, "in this country or abroad".
She said: "Those placed on the sex offenders register will have their foreign travel monitored and we will alert other countries to their presence where necessary."
Whether a particular sex offender would be banned from travelling overseas was a matter for the courts, she said.