NHS worker Jakki Smith, 58, only discovered she was not entitled to the statutory award after the death of John Bulloch, her partner of 16 years.
The fixed sum of £12,980 is paid out if a person dies as a result of negligence - but only to spouses or civil partners.
Father-of-three John Bulloch, a retired prison governor, died aged 66 in 2011 after an infection was missed by medics and spread to his brain.
Jakki Smith, a mother of one from Chorley in Lancashire, who lived with Mr Bulloch for 11 years prior to his death, has now launched a landmark legal fight to improve the rights of unmarried couples.
She said: “Neither John or I were anti-marriage - we had both been married before - but that didn’t mean we weren’t in a meaningful, permanent relationship and totally committed to each other.
“We’re expected to live by society’s norms, but society’s norms are changing.
“Marriage rates are falling year on year, yet the law seems stuck in the past.”
She added: “John wasn’t one for ceremony, but he did believe in everyone being equal.
“It’s a point of principle. This is something I can fight for in his memory because I know he would agree with me.
“There are so many couples who don’t want to get married, but that doesn’t mean they shouldn’t be treated the same.
“I’m not doing it for myself, I’m doing it for them.”
In the case against the Secretary of State for Justice, which is due to be heard later this year, lawyers will argue that the current legislation violates articles eight and 14 of the European Convention on Human Rights in that it discriminates against Ms Smith on the grounds of her marital status and her right to respect for private and family life.
In Scotland, the law on bereavement damages already extends to cohabitees as well as other immediate family members such as parents or children.
Zak Golombeck, a human rights lawyer from Slater and Gordon, who is representing Ms Smith, said: “The loss of a long-term partner is absolutely devastating, yet for unmarried couples there is still no recognition of that loss in the eyes of the law.
“What we are seeking is a reinterpretation of the current legislation to include those couples who have lived ‘as husband and wife’ for more than two years.
“A wedding ring is no longer the only sign of commitment in a modern relationship and the law should be changed to reflect that.”
The case is the latest to challenge the law over unmarried couples.
Mother-of-four Siobhan McLaughlin recently claimed that her human rights had been breached when she was refused widowed parent’s allowance following her long-term partner’s death. A judge in Northern Ireland found in her favour.