The investigation by the Chief Whip, Nick Brown, follows newspaper claims that Mr McMaster left a letter in which he accused two senior Labour colleagues of a whispering campaign against him which may have been a factor in his death.
Reports said that the letter named Tommy Graham, Labour MP for Renfrewshire West, and Lord (Don) Dixon, the former Labour deputy chief whip. Mr McMaster was said to have asked that the letter be published.
In a brief statement last night, Mr Brown said: "I do have a copy of the letter. The Prime Minister has asked me to investigate. I am investigating."
Neither Lord Dixon not Mr Graham were available to comment last night, but Lord Dixon was quoted in the Sunday Times as denying spreading malicious rumours. He said he and Mr McMaster had not met for about a year but had always got on well before that.
"I have never heard the suggestion that he was gay and it is the last thing I would suggest or think. I will be flabbergasted if I have been named in the note. Gordon was a good comrade. The last thing I would do is talk about him in that way," he told the paper.
The newspaper claimed the letter was addressed to Mr McMaster's parents, the Prime Minister and Irene Adams, Labour MP for the neighbouring constituency, and that the author appeared to have been in a disturbed state of mind.
The 37-year-old MP for Paisley South was found dead in the garage of his home in Johnstone, Strathclyde, last Monday.
Friends at Westminster said the 18-stone MP, who held the seat for Labour with a majority of 12,750 on 1 May, had been suffering from myalgic encephalitis, the chronic fatigue syndrome, and had been depressed for some time. His funeral took place at Paisley Abbey in Renfrewshire last Friday.Reuse content