Portugal has refused permission for a Dutch "abortion ship" to enter its territorial waters.
Portugal only allows abortion in exceptional situations
The government said the group operating the trip planned to hand out pills to end early unwanted pregnancies - a procedure prohibited in Portugal.
In Portugal, abortions are only allowed in exceptional circumstances, such as when the mother's life is at risk.
The ship has stopped in other Catholic countries with restrictive abortion laws such as Poland and Ireland.
The vessel leased by pro-choice group Women on Waves is staffed by a doctor, a gynaecologist and a nurse.
To circumvent Portuguese law, volunteers had planned to hand out pills which induce abortion once the ship was back in international waters where Dutch laws would have applied to the
Portuguese Secretary of State for Maritime Affairs Nuno Fernandes Thomaz cited legal and health reasons for turning the ship away:
"It is a question of legality and not of morality.
"If we accept that third parties come and violate our laws it would become much more difficult to impose authority amongst the Portuguese."
He pointed out that the abortion pill is banned in Portugal.
One of the volunteers on board the ship told AFP news agency by telephone that they were "trying to figure out" what they could do.
Up to 40,000 illegal abortions - some of which are fatal - are carried out in Portugal every year, according to family planning agencies.
Women are periodically taken to court for having unwanted pregnancies illegally terminated.
Many Portuguese favour changing the law to make getting an abortion easier, according to opinion polls.
But in the spring, Portugal's centre-right administration used its majority in parliament to vote down proposals by left-wing parties which would have loosened abortion legislation.