The former Conservative minister David Mellor said it was "good news" that the police seem to be taking leaks to the media seriously.
"What happened in the Mitchell case was a serious breach of duty with not only the full details of police reports being leaked, but the actual documents being handed over to newspapers," he said.
"This was disgraceful. The question has to be asked, what took them so long?"
Mr Mellor said he would be watching "with interest" to see whether the arrest would be taken further, and whether it would change "deeply ingrained" behaviour among the police.