Porton Down veteran Fred Nunn is staying tight-lipped about a potential compensation offer and apology from the Ministry of Defence.

The 77-year-old grandfather, of Gilstead, Bingley, who claims he was duped into taking part in chemical warfare experiments, said he and others seeking compensation had been told by advisors to stay silent.

So he was stunned, he said, to switch on the news yesterday and hear about the Ministry's plans to offer a £3m payout.

According to reports, about 90 per cent of veterans have agreed to accept £8,300 each and an apology in an out of court settlement.

He said: "I couldn't believe my ears. We'd all been advised by our solicitors to stay quiet until any offer was signed, sealed and delivered.

"It shouldn't have come out like this. Everyone was told not to say a word. I don't want to say anything until it is all sorted out."

An MoD spokesman said: "The issue of whether any mediated settlement would be acceptable to the veterans is entirely a matter between the veterans and their solicitors. The parties are engaged in a mediation process which is not yet concluded, the details of which are confidential. It would be inappropriate to comment further at this stage."

It was a veteran from Hampshire who broke the silence and said although he had accepted the deal, the MoD was still refusing to pay out because others still had not signed the agreement.

Mr Nunn was a 22-year-old leading aircraftsman when he volunteered to be a human guinea pig for what he thought was cold remedy tests at Porton Down - the chemical and biological research centre in Wiltshire.

It was only in 2004, at an inquest into the death of fellow volunteer Ronald Maddison, that Mr Nunn discovered he and hundreds of others had been exposed to the deadly nerve gas Sarin.

The inquest ruled 20-year-old Leading Aircraftsman Maddison had been unlawfully killed and his family finally got £100,000 in compensation.

In a previous interview, Mr Nunn told the Telegraph & Argus: "I'd gone in with just my uniform, others had gas masks. They told me to breathe normally. I didn't see the gas come in but I heard it. My eyes started watering, then my chest tightened. I thought I was going to snuff it."

No consent forms were ever signed, said Mr Nunn, and although all of the volunteers were promised 15 shillings and a week's leave in return. He only ever got a 48-hour pass.

After the 2004 inquest Mr Nunn and other former Porton Down servicemen who suffered ill-health joined a veteran's group to fight for justice.