A heterosexual couple will request a civil partnership – available only to same-sex couples in the UK – at a register office in London tomorrow, to take a stand against a system which they say "segregates couples according to their sexuality".

Tom Freeman and Katherine Doyle, both 26, will go to Islington register office to ask for the partnership, although they expect their request to be turned down. Their attempt to enter into a civil partnership is part of the Equal Love campaign, which aims to challenge what activists say is a twin ban on gay marriages and heterosexual civil partnerships.

"We want to secure official status for our relationship in a way that supports the call for complete equality and is free of the negative, sexist connotations of marriage," said Freeman, who works as an administrator.

"We are taking this stand against discrimination and in support of legal equality for everyone, regardless of sexual orientation.

"The 'separate but equal' system which segregates couples according to their sexuality is not equal at all. All loving couples should have access to the same institutions, regardless of sexuality. There should be parity of access."

The Equal Love campaign is organised by lesbian, gay, transgender and bisexual group Outrage. Eight couples will apply for marriages or civil partnerships over the next eight weeks. Campaigners say denying gay and straight couples the same treatment contravenes the Human Rights Act.

The first of the eight couples, Sharon Ferguson, a pastor of the Metropolitan Community Church, and her partner Franka Strietzel applied for a marriage licence at Greenwich register office, south-east London, but were turned down.

Freeman and Doyle will be joined at the register office by the human rights campaigner, Peter Tatchell, who said today that preventing straight couples from entering into civil partnerships was "heterophobic".

"It is discriminatory and offensive," he said. "I want to see it ended, so that straight couples like Tom and Katherine can have the option of a civil partnership. I applaud their challenge to this unjust legislation."

Freeman and Doyle had applied for a civil partnership last year, again in Islington, but were turned down. The council said today that its position remained the same.

"Like all councils, we must follow the requirements of the Civil Partnership Act 2004, which states that to qualify for a civil partnership, couples must be of the same sex," it said. "Whilst we can't legally accept the proposed civil partnership, we would be delighted to offer the couple a civil marriage."

The campaign has some parallels with a court case in Austria, where a couple are seeking to become civil partners rather than husband and wife.

Austria introduced the registered partnership earlier this year, but it does not grant the same legal rights as marriage. The Austrian couple concerned filed for an eingetragene partnerschaft but were rejected. After taking their case to the two highest courts in Austria, the Verfassungsgericht and the Verwaltungsgericht, they are awaiting a decision.

Their lawyer, Dr Helmut Graupner, said he was willing to take the case to the European court of human rights if they were unsuccessful. "You could call this sexual apartheid," he said. "There's marriage and civil partnership, one excluding straight couples, the other excluding homosexual couples. We want people to have a choice."