The decision is likely to delight the Conservatives, providing a platform during the by-election for them to continue John Major's attack on Labour's devolution plans.
Last week the Prime Minister was again arguing that Labour's plans for Scotland and Wales would lead to the break-up of the United Kingdom.
Opposition to the Assembly came at a meeting just before Christmas of the 30-strong General Management Committee of the constituency when faced with Labour's consultation document on how the proposed Assembly should be elected, financed and run.
The committee agreed to fill in the questionnaire but also to send in a letter underlining the constituency's long-standing opposition to the idea.
"It was a unanimous decision to do that," said Tony Wilkins, a delegate to the constituency and Labour chairman of the shadow unitary local authority for the area which is due to take over from Gwent County Council next year.
An Assembly would be divisive, he argued, would lead to more pressure for use of the Welsh language when only 2 per cent of the country is Welsh speaking, and would be costly.
"If Labour gets in and sets up an Assembly we will within two years have another major upset. This new authority is going to cost millions in expense so what would a Welsh Assembly cost?"
Islwyn has a history of opposition to a Welsh assembly with Mr Kinnock a somewhat reluctant convert to the idea when party leader.
Meanwhile, Labour divisions over a Scottish Assembly deepened yesterday when Tam Dalyell, the longest-serving Scottish Labour MP, criticised bitterly the party's devolution plans as "absurd nonsense" which could cost Labour the next general election.
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