Labour MP Paul Flynn has attacked housing charity Shelter Cymru because he says it is ignoring the progress made in tackling housing problems.
Writing in his blog, the MP for Newport West said the charity operated "a fatwa against good news".
He said it had invited him and other MPs to a "sob-in" at Westminster and he published official figures suggesting homelessness in Wales had decreased.
Shelter Cymru said it was keen to work with MPs on homeless issues.
The charity's director, John Puzey, said he had posted the invitation letter to Welsh MPs onto the Shelter's website and denied it was a "sob-in".
He said the Shelter Cymru invited MPs every year to consider housing and homeless issues in Wales as part of a "constructive discussion".
In Mr Flynn's blog, he also said many pressure groups were guilty of "bloated empire-building" but pointed out that some were also well-managed and did marvellous work.
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Quoting figures published by the Welsh Assembly Government last week, Mr Flynn said the number of Welsh homeless in priority need between June and September 2007 fell by 9% in comparison to statistics for the same period in 2006.
The number of households in temporary accommodation fell by 6% and those in bed and breakfast fell by 46%, said Mr Flynn.
'Cynical and pessimistic'
"Strange not a mention of this spectacular good news from Shelter Cymru," he says in his blog.
"Of course, there are cases of distress which involve lack of decent housing but the majority of those described as homeless have problems of addiction from alcohol and other drugs, mental health difficulties and family breakdown.
"Crudely lumping them together under the misleading title of 'homeless' inhibits progress."
But Mr Puzey said the charity had publicly acknowledged a number of developments in housing and homelessness and described Mr Flynn's comments about pressure groups as "an extremely cynical and pessimistic view of what charities like ourselves do".
He said: "Overwhelmingly, our message is, and our press work has been, positive and optimistic.
"I think Mr Flynn should look at these things before he makes these unsubstantiated comments.
"I have to say I don't think Mr Flynn fully understands the issues.
"It's very important to us, if we're going to work with politicians and other agencies, to provide a balanced view of what's going on.
"We have acknowledged the progress in addressing homelessness and housing in Wales but we also have a responsibility of saying there's a lot more to be done."
Mr Flynn was elected in 1987 and is known for his controversial views on drug legislation and nuclear weapons.