The following correction was printed in the Guardian's Corrections and clarifications column, Monday 22 June 2009

In the article below about fertility clinics in Spain we said insemination treatment has become more difficult in Britain since sperm donors lost the right to remain anonymous in 2005 and that, after the law changed, the numbers of sperm and egg donors fell dramatically. Figures published by the Human Fertilisation and Embryology Authority show that, although the number of new egg donors declined in the two years after donor anonymity ended, the number of new sperm donors increased during that period. Overall, the numbers of new egg and sperm donors registering at clinics in Britain fell.

As niche package tours go it is one of the most original and precisely targeted. As of this week, British lesbians are being invited to dig into their pockets, catch a flight to the Spanish costas and come back either pregnant, married or both.

The offer comes from a fertility clinic and a gay and lesbian travel agency, which have launched joint package tours to what has now become one of the most socially tolerant countries in Europe.

Together they have spotted a growing trend among British lesbians, who increasingly travel to clinics in liberal Spain for insemination treatment, which has become more difficult in Britain since sperm donors lost the right to remain anonymous in 2005.

"We noticed the increase in British women coming to us as soon as the law was changed," said Dr Rafael Bernabeu, founder of the Instituto Bernabeu clinic in Alicante, eastern Spain. "Here we can still offer that anonymity, so people are coming to us."

Bernabeu said his clinic saw 30 British women a month. About 40% of British women seeking donor insemination were single and many were lesbians. "We don't ask questions about people's sexuality, so I can't give exact figures," he said. "But often they come with same-sex partners or simply tell us that they are lesbians."

He said that couples, gay and straight, who wanted fertility treatment were often worried about the effect on a child of knowing that they could track down their missing biological parent. "Many couples in this situation do not want problems of that kind," he said.

The guarantee of anonymity means Spanish clinics have few problems finding sperm and egg donors, whose numbers fell dramatically in Britain after 2005 as people worried about getting a knock on the door in 18 years' time.

Unlike some other countries in Europe, Spain does not require those wanting fertility treatment to be married. "As long as you are in control of your mental faculties, then you can do it," said Bernabeu. "The question of your sexual orientation or whether you are married has nothing to do with it."

Spanish law obliges doctors to seek as close a physical match as possible between the parents and the donor.

British women take day trips to Alicante to start the treatment, continue with it at home and then make a day trip for insemination or implantation of an egg. "Sometimes, however, they need to be here for several days, so we thought it would be a good idea to approach the new gay and lesbian travel organisation in Alicante," Bernabeu said.

"Two women travelling to Spain so that they can have a baby are doing something that is very special to them," said María José Rico, of Rainbow Travel. "We want to make the experience as pleasurable as possible. The treatment does not require you to rest a lot, so we want to offer people things to do and places to stay that will make it a real holiday as well," she said.

Hotels with views over the Mediterranean, evening outings in yachts and massages to help with pre-treatment nerves would all be on offer, she said.

Spain, which introduced gay marriage four years ago, has shed its Roman Catholic-inspired suspicion of same-sex couples, Rico said. "Alicante is very cosmopolitan. We live in the 21st century and nobody finds it strange to see two women here with a child," she said. "In fact if people want to come back here to celebrate the child's birthdays, we'd be happy to arrange that as well."

Rainbow also offers to help lesbian couples who want to marry in Spain. "First of all we help them bear a child and then we can help them marry – as long as they meet the legal requirements," she said. "That makes them feel psychologically more like a family."

More than 12,000 same-sex Spanish couples have married in the last four years. "If they are both foreigners then one of them has to be resident in Spain," said Rico. It is relatively easy to register as a resident in Spain and formally meet the requirement. The marriages are valid only in Spain and in other countries that accept same-sex marriages.

Britain and Spain have the most liberal assisted fertility regimes in Europe. Bernabeu said his clinic also received many Italian patients – where sperm and egg donation is illegal – and from northern European countries that require women to be married. He was also beginning to get patients from the US. "We cost a lot less," he said.

The process is not cheap, however. Prices for egg implantations in Spanish clinics start at about €7,000 (£6,000) and go above 12,000. Donor insemination is generally cheaper, starting at about €1,500.