Relatives of soldiers killed in the Iraq war are appealing against the government's refusal to hold an independent inquiry into its legality.
Mr Keys and Mrs Gentle both stood in the general election
Lawyers have lodged papers at the High Court seeking a judicial review.
Reg Keys, whose military police officer son Thomas died in 2003, and Rose Gentle, whose son Gordon was killed in 2004 are among the applicants.
The government has dismissed the challenge made under human rights law saying it is "fundamentally flawed".
The families say the UK is obliged to hold an inquiry if involved in the use of lethal force.
They have also asked judges to rule on the remit of any inquiry.
The group Military Families Against the War said the hearing is expected to take place in September.
Mr Keys, from north Wales, whose 20-year-old son was one of the six Royal Military Police killed by a mob near Basra in June 2003, unsuccessfully stood against Tony Blair in the general election.
But he managed to poll 10% of the vote in the Sedgefield constituency standing on an anti-war ticket.
Flanked by Mrs Gentle and solicitor Phil Shiner outside the London court, Mr Keys said the group would not be taking the action "if weapons of mass destruction had been found in Iraq".
"We most strongly feel our sons were sent into a conflict not backed by international law or the United Nations," he said.
"Our boys were fully prepared to lay their lives down to defend their country. They were sent to war on a falsehood, against a background of propaganda of weapons of mass destruction."
Mrs Gentle, from Pollok, Glasgow, was also defeated in the election after standing against Armed Forces Minister Adam Ingram.
Her 19-year-old son was killed by a roadside bomb in Basra while serving with the Royal Highland Fusiliers.
"We intend to fight this until we get to the truth," she said. "We want to get the truth for our boys."
The call for a judicial review is supported by the families of 17 soldiers who died in the conflict and one who killed himself after returning to the UK.
The families want the prime minister, the Attorney General, Geoff Hoon, who was the defence secretary at the time, and Foreign Secretary Jack Straw to be cross-examined about the conflict.
In May they delivered a letter to Downing Street demanding an inquiry.
Treasury solicitors told the families: "The legality of the decision to take military action in Iraq has no bearing on the circumstances which led to their deaths".
It had previously been made clear that decisions on military action abroad are not subject to review under the European Convention on Human Rights, they added.