Home Secretary Jacqui Smith has come under pressure to explain why police entered the House of Commons office of a Tory MP and demanded constituency correspondence.
Daniel Kawczynski said he was "appalled" that a police officer had entered his office as part of a probe into a series of letters containing white powder sent to the Commons.
The Metropolitan Police said an officer entered Mr Kawczynski's Westminster office "by appointment", and the MP had agreed to hand over a letter allegedly matching the handwriting of those under investigation.
The incident led to calls for Ms Smith to make a statement to MPs and for Commons Speaker Michael Martin to investigate.
Mr Kawczynski told the Commons: "They said they were investigating an important case with regards to correspondence that had been sent to ministers and wanted to see handwriting samples of writing that people had written to me.
"I was extremely appalled that the officer can behave in this way, to enter a Member of Parliament's office with no warrant and to demand constituency correspondence."
The Shrewsbury and Atcham MP added: "To my great embarrassment and eternal shame, I was so weak I handed over the letter they demanded from my constituent."
He said he found it "disgraceful" that police had entered his office after the controversy over the arrest of shadow immigration minister Damian Green.
Mr Green's Commons office was searched as part of a Home Office leak inquiry in November last year. Following that incident, Mr Martin pledged that police would never again be allowed access to an MP's office or parliamentary papers without a warrant and the personal approval of the Speaker.
The Home Office declined to comment on the incident, saying it was a matter for the police. A spokesman for Mr Martin's office confirmed he had been made aware of the latest incident after Mr Kawczynski raised it in the Commons on Wednesday night.