The company has defended the £8,000 payment, claiming it was simply sponsorship of a reception for MPs at last year's Labour Party conference in Blackpool.
However, a report in The Mail on Sunday has called this into question because the Electoral Commission's website shows the gift was made on New Year's Eve, three months after the conference.
The donation could also be seen as controversial because the company is waiting to hear whether its merger with Granada will be referred to the Competition Commission.
A competition inquiry is thought likely because the merger will see ITV control more than 50% of the TV advertising market. ITV's advertising share has dipped to 52%, ITV joint managing director Clive Jones said last week.
Analysts are reported to have called it unusual for big companies to make political donations, and Carlton's ITV partner Granada does not make political donations.
This is not the first time Carlton has made a political donation. In November 2001, it made a £2,500 donation to the Conservatives as sponsorship of a reception.
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This article was first published on brandrepublic.com