Swiss lawmakers voted on Friday to legalise same-sex marriage and allow transgender people to change their legal gender by making a declaration, marking a major step forward for LGBT+ rights in the country, campaigners said.

The gay marriage law is likely to be tested in a nationwide referendum next year before it takes effect, but rights activists said they expected it to secure popular support.

"This is not only a milestone in the fight for the rights of the Swiss LGBT population, but also an important victory for their dignity, their acceptance and their inclusion in society," Marriage For All, a campaign group, said on its website.

Switzerland had lagged behind other parts of western Europe on LGBT+ rights, with political institutions tending to be more conservative than the public.

A law protecting lesbian, gay and bisexual people from discrimination was only passed earlier this year.

Under the legislation approved by parliament on Friday, trans people will be able to change their gender on identity documents by making a declaration at civil registry offices.

That would make Switzerland the eighth European country to allow trans people to legally change gender without the involvement of a doctor or a court, in what is known as "self-ID", according to advocacy group Transgender Europe.

The minimum age for changing gender legally without parental consent was set at 16, drawing criticism from trans rights advocates.

They noted that at present name changes and medical treatments related to transitioning gender do not require parental consent for the under-16s.

"On the one hand, we're super happy there will be this legal gender recognition based on self-determination, in a very quick and simple procedure," said Alecs Recher, the head of legal services at Transgender Network Switzerland, an advocacy group.

"But, on the other hand, it's a major step back for those under 16," he told the Thomson Reuters Foundation,

Under current rules, both children and adults have to apply to a court to change gender, said Recher, whose organisation supports trans people through the legal procedure, which can cost up to 1,000 Swiss Francs ($1,130).

Switzerland will become the 29th country to allow same-sex marriage, if the law - which will also let lesbian couples conceive using sperm donation - comes into force.

Opponents have 100 days to collect the 50,000 signatures needed to trigger a referendum.

A survey commissioned by a gay advocacy group Pink Cross in February showed more than 80% of Swiss support same-sex marriage, suggesting the law would take effect even if opponents garner enough signatures to force a referendum.