An inquest ruled Ronald Maddison was unlawfully killed

Ronald Maddison, from County Durham, died in 1953 after being exposed to sarin at Porton Down in Wiltshire.

Solicitor Alan Care said compensation for the family was due to be discussed on Friday. Talks will also be held on possible pay-outs for other veterans.

"We see this as a highly significant moment," Mr Care said.

Mr Maddison died after Sarin nerve gas was used on him during apparent tests for the common cold at the Porton Down base.

We feel the Porton Down veterans should be treated in the same way as Canadian veterans Solicitor Alan Care

An initial inquest, which was held in secret, reached a verdict of death by misadventure, but this was quashed in 2002 by Lord Chief Justice Lord Woolf.

A second inquiry was launched in May 2004 and it ruled in the following November that Mr Maddison was unlawfully killed.

Although the government has said it would challenge this inquest verdict, Defence Minister Ivor Caplin has said any claim for compensation from the airman's family would be "considered favourably".

Mr Care said lawyers representing the Maddison family would meet with Ministry of Defence (MoD) representatives and treasury solicitors at a meeting in London on Friday.

This would be followed by a second meeting in which possible payments for 550 other veterans who took part in Porton Down tests would be discussed, Mr Care said.

'Not going away'

The Canadian government has announced it is to make one-off payments of $24,000 (�10,300) to veterans who volunteered for similar tests "in recognition of their services to Canada".

It is estimated the payment programme could cost the Canadian government $50m (�21.6m).

"We feel the Porton Down veterans should be treated in the same way as Canadian veterans," Mr Care said.

But he stressed claims for "ex gratia" payments were separate to any possible compensation claims from veterans who say their health has suffered as a result of the tests.

Gerry Morris, from Deal, in Kent, is to attend the meeting in London to ask questions on behalf of the Porton Down Veterans' Support Group.

Mr Morris, who took part in tests in Porton Down in 1958, said: "It has taken several years to get to this point.

"Hopefully the government will start to look honestly and positively at this problem and accept that it isn't going to go away."

An MoD spokesman said he could not comment on any meetings to discuss compensation payments as these were private matters.