The Liberal Democrats, who have accused both the other main parties of abusing TPS rules by making marketing calls, insist that their complaint is still an ongoing matter with the ICO.
The party complained to the ICO at the end of March and chairman Matthew Taylor said Labour was "acting no better than a crooked double-glazing salesman".
More than 700 people are believed to have contacted the Liberal Democrats to complain about receiving the calls.
Yesterday, a Liberal Democrat spokesman slammed the ICO, saying that if it had decided not to proceed with its investigation it would give a green light for everyone to break the law.
However, an ICO spokeswoman confirmed that it was not taking any action against Labour or the Tories.
The Liberal Democrats spokesman said the party had received a letter from the ICO and had relayed fresh concerns about the issue in phone calls. One of the concerns was that the Conservatives had twice called Annette Brooke, its MP for Mid Dorset and North Poole who is TPS registered, with automated marketing messages.
The spokesman also accused TPS complaints line staff of giving out false information. The Liberal Democrats have been encouraging people to complain to the TPS and the ICO, and according to the spokesman the TPS complaints line has told callers that political parties were not covered by the TPS.
TPS manager Scott Flower said staff had not given out false information and put the problem down to confusion about the rules regarding phone calls for political purposes.
Flower also revealed that the TPS had written to Labour and the Tories to warn them about calling TPS registrants, as well as reporting the breaches to the ICO, but was unable to say how many complaints the TPS had received.
The TPS issued a statement last Friday to clarify the legal position on political telemarketing.
It said that it was a legal requirement under the Privacy and Electronic Regulations 2003 that political parties should not make unsolicited calls to promote their party or solicit support to a person registered on the TPS unless the person had given their consent.
But it was perfectly legal for a political party to conduct research by telephone, although in carrying out research it is vital that political parties ensure they do not ultimately solicit support.
Colin Lloyd, chairman of the Telephone Preference Service, said: "The telemarketing activity of political parties has been causing confusion amongst the public and we felt it was important to clarify this position to meet the needs of our subscribers and ensure the continued effectiveness of the service."
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This article was first published on brandrepublic.com