Fourteen servicemen were killed when a Nimrod exploded in mid-air in 2006
Defence Secretary John Hutton has been served with a writ by relatives of two of 14 men who died when a Nimrod plane exploded in mid-flight in Afghanistan.
The action accuses the MoD of negligence, failing to minimise risk and a breach of the right to life.
A holding writ was served in September, the full form had to be in by January.
In May, a coroner ruled the Nimrod fleet had never been airworthy. The MoD said it had already planned to compensate families of the servicemen.
The coroner also said the fleet, based at RAF Kinloss in Moray, should be grounded.
The Nimrod spy plane exploded on 2 September 2006, shortly after undergoing air-to-air refuelling.
The blast was caused by fuel leaking into a dry bay and igniting on contact with a hot air pipe.
The legal action is being brought by the families of Sgt Ben Knight, 25, and Flt Lt Steven Swarbrick, 28.
Graham Knight, Ben's father, said: "It is about accountability.
"When you look at things that have happened recently like the Baby P case, it was tragic that child died and there were resignations and sackings.
"In this case there was an aircraft that was not airworthy, 14 men died and as far as we know not one person has had action taken against them.
"We want to know who was responsible, who should have made sure it was airworthy and failed in their duty of care."
Later, Mr Knight told BBC News: "It's like if it had been a hit-and-run, we don't know who the driver is.
"He [Sgt Knight] would have been willing to lay down his life for the country if it was insurgents that killed him, but it wasn't, it was incompetence."
Mr Knight added that Christmas was a particularly difficult time of year to deal with the loss of his son.
He said: "Every day is hard, but Christmas is harder... on Christmas Day you always ring round your relatives and say happy Christmas and he's just not there any more to wish happy Christmas to."
The issuing of the writ is the first time the department has faced a legal challenge under the European Convention of Human Rights.
If it is successful it could pave the way for other families to sue the government over the death of loved ones in Iraq or Afghanistan.
The MoD has said new procedures mean the Nimrod aircraft is safe.