Jim Smith, the co-ordinator of Paisley North constituency Labour Party, said he had been assured the inquiry by Nick Brown, the Government Chief Whip, would be thorough and wide-ranging.
Mr Smith also said there would be quick and decisive action against MPs who were found to have acted "inappropriately".
Tommy Graham, named along with Lord Dixon in the suicide note by Mr McMaster, has been criticised by Scottish Labour sources for a recent interview repeating allegations against Mr McMaster that he had a drink problem.
Mr Graham, MP for Renfrew and Inverclyde, who is expected to be interviewed by Mr Brown, is seeking to clear his name from the allegations in the suicide note that he was part of the whispering campaign against Mr McMaster, but his interview was described as "not very clever".
The pressure on the Labour Party to take disciplinary action was increased when the Scottish National Party suspended one of its councillors for allegedly spreading smear stories about Mr McMaster.
Mr Smith said on BBC radio he had been assured there would be "no cover up" following a report yesterday in The Independent that friends of the dead MP feared the inquiry would be a whitewash unless it delved into allegations of links in Scotland with drugs gangs and organised crime.
He said: "I have been assured there will be no cover up. Every aspect of this will be investigated fully and if there is anybody in the Labour Party guilty of inappropriate behaviour that will be dealt with most severely.
"This inquiry must be absolute. There must be no doubt left in anyone's mind that the Labour Party has looked into every aspect of this dreadful affair. I have no doubt that the Chief Whip will do this," he added.
Mr Brown cancelled his holiday to go to Scotland, but faced growing demands for the inquiry to be widened following the disclosures in The Independent.
Mr Brown met the parents of the dead MP, William McMaster and his wife Allison, on Monday to discuss the suicide note. He also met Leslie Quinn, a senior official of the Scottish Labour party.
The content, location, and outcome of Mr Brown's meeting with the parents was not disclosed.
But a source confirmed the MP's suicide note - in which he is believed to have bitterly complained of a smear campaign against him - was discussed.
"That was the main purpose of the meeting," said the source.Reuse content