He has told the BBC that the controversial bill was being “jostled for space” in the legislative programme.
It was originally due to go to Parliament before the summer break. He says he now expects it to occur “later”.
The reforms are being opposed by unions and many Labour MPs, with a number of Labour backbenchers signing a Commons motion critical of the plan.
Lord Mandelson has also said that the poor state of the economy is another factor delaying the process, according to reports.
The delay follows the Government’s assurance at the beginning of the month that would remain committed to selling a stake of the postal service, despite speculation that it is considering ditching its plans after failing to secure a high enough bid.
CVC Capital Partners, the private equity group, is reportedly one of the frontrunners to take a 30% slice of Royal Mail, but its offer of less than £2bn is much lower than the Government’s original asking price.