The Ministry of Defence is preparing to award £3m in compensation to 360 veterans of nerve gas experiments at Porton Down, the BBC has learned.
Hundreds of servicemen took part in the Porton Down experiments
The ex-servicemen say they were duped into taking part in what they thought were cold remedy tests at the Wiltshire research centre in the 1950s and 1960s.
About 90% of them have agreed to accept payment of £8,300 each and an apology.
But others say the MoD is bullying them by insisting everyone accepts the offer before it pays out. It denies this.
The long-running legal claim against the MoD has been pursued by ex-servicemen who were recruited as volunteers to test cold remedies at the government's Porton Down laboratories.
In fact, many of the soldiers were given forms of the sarin nerve agent developed by the Nazis in World War II.
Many say they have suffered a lifetime of ill-health, including skin diseases and psychological problems, as a result of their exposure to the gas.
Now, after a campaign lasting many years, the MoD is offering £3m compensation and an apology in an unprecedented out of court settlement.
The exact terms of the apology are still under discussion and the MoD says they will remain confidential until negotiations are concluded.
Some of the men have refused the MoD's offer, saying the flat fee of £8,000 is insufficient.
One veteran, Derek Shenton, from Hampshire, said although he had accepted the deal, the MoD was still refusing to pay out because others had refused to sign the agreement.
Leading Aircraftman Ronald Maddison died at the base in 1953
He said: "There was very high pressure to sign - threats basically: 'Take it or leave it'.
"But once the Ministry of Defence got my signature, they came back and said 'because there's these various people who have decided to go missing, we are not going to pay you until we have got their signatures'.
"The whole thing is disgraceful."
The MoD has denied any suggestion of coercion.
But it has faced increasing calls to act since an independent review looking at the ethics of the Porton Down experiments.
Additional pressure has been brought by the 400-strong Porton Down Veterans Group, which has taken legal action over ill health suffered by its members.
Between 1939 and 1989, hundreds of servicemen took part in experiments at Porton Down.
Only one other Porton Down case has resulted in compensation.
That was for 20-year-old aircraftsman Ronald Maddison, of Consett, County Durham.
Back in 1953, he volunteered to take part in what he believed to be a test for a cold cure.
Within an hour of having sarin dabbed on his arm at the Salisbury Plain laboratory, he was dead.
The MoD paid £100,000 in compensation to Mr Maddison's family but only after mounting an unsuccessful legal challenge against a 2004 inquest into his death, which ruled he had been unlawfully killed.