The allegations by Llew Smith, Labour MP for Blaenau Gwent, were confirmed in a letter to the Government Chief Whip by Bernard Assinder, the chairman and Labour leader of his local council.
Mr Assinder told the Chief Whip that he had received a telephone call from Huw Roberts, the special adviser to Ron Davies, the Secretary of state for Wales, warning that there could be "difficulties" for the council, if Mr Smith campaigned against Welsh devolution.
The letter is likely to cause embarrassment for Tony Blair, with the Conservatives in full cry for a Commons investigation into alleged threat to freedom of speech by MPs.
It shows that the Welsh Office was threatening to refuse meetings with the Welsh Labour MP, if he campaigned against devolution.
The letter, written earlier this month, also appeared to contradict another letter by Mr Assinder, released by Downing Street officials on Thursday, denying the MP's allegations.
Mr Assinder told the Chief Whip that Mr Davies's adviser had expressed concern that Mr Smith "was intending to engage in a campaign against devolution. He indicated that this could have a damaging effect upon Blaenau Gwent, in that, if Llew was disciplined because of his actions, there would be difficulties with the Welsh Office holding discussions with Blaenau Gwent, especially with Llew being present.
"We could be placed in a position where we would have to hold meetings without Llew which was obviously not the normal practice.
"He said it might therefore make sense to talk to Llew about his intentions regarding devolution."
A total of 20 Tory MPs yesterday tabled a Commons motion to embarrass Mr Blair by calling for an inquiry by the Committee on Standards and Privileges.
But Donald Dewar, the Secretary of State for Scotland, sought to defuse the row by indicating that he could take a relaxed view of any Labour MP who campaigned against devolution plans north of the border.
"If there are individuals who take a different point of view, so be it ... I'm very, very happy to let things go and to have a decent and genuine debate," he said. But Mr Dewar stressed that he did not expect such dissent to arise.Reuse content