The MP also said that the investigation related to a letter sent to Ed Balls. It contained a white powder that turned out to be flour.
A Conservative MP has alleged that police entered his Commons office without a search warrant demanding to see constituency correspondence.
Daniel Kawczynski said he found the episode "disgraceful".
The Shrewsbury and Atcham MP said he had handed over a Shrewsbury-postmarked letter related to an inquiry over "white powder" sent to a minister.
But Scotland Yard said a Parliament-based police officer had entered the office "by appointment".
Mr Kawczynski said it was to his "eternal shame" that he had complied by handing over the letter.
Kawczynski has previously been in the news after accusing the BBC of anti-Polish racism and claiming that Otis Ferry was being held as a political prisoner.
Later. Spy Blog has the extract from Hansard where Kawczynski raised the matter today:
Michael Crick said on Newsnight that the general view on the Tory benches was that Kawczynski had got rather overexcited about the matter. Either way, it is hard to demur from Spy Blog's verdict that he is a "prize idiot" for handing over the letter.
On a point of order, Mr. Deputy Speaker. I am extremely shocked about what I am going to say. I was about to make my speech in the debate on savers when I received a note from my office saying that there was a police officer there, demanding to see correspondence.
The police were already present in my office and I went to see them after making my speech. They said that they were investigating an important case with regard to correspondence that had been sent to Ministers and wanted to see handwriting samples from people who had written to me. I am appalled that officers can behave in that way--entering a Member of Parliament's office, with no warrant, and demanding constituency correspondence.
To my great embarrassment and eternal shame, I was so weak that I handed over the letter from my constituent that they demanded. I will have to live with that, but I am extremely embarrassed about it. After everything that has happened to my hon. Friend the Member for Ashford (Damian Green), it is disgraceful that this is happening and I urge you to investigate.