Gordon Brown refused to bow to pressure from unions and Labour MPs to drop plans to sell off part of Royal Mail, warning critics there was no other way to secure postal services.
The Prime Minister, who was booed and jeered by protesters angry over the proposals on his way to make a speech in Bristol, said he would listen to the concerns.
But at a time when there was a huge demand for funds to get the country and public services across the board through the recession, there was no option but to seek outside help, he told them.
"I know this is difficult ... we will continue to talk and address the concerns people have. But I assure you the Government is looking at how we can best prepare for the future, not only of the Royal Mail but the future of all public services in the country," he told the Labour's National Policy Forum.
Business Secretary Lord Mandelson published the Bill paving the way for a partial sale this week, insisting it would remain in public hands even though up to 30% could be sold to a foreign firm.
Under the plans, the taxpayer will take responsibility for the deficit-ridden pension fund but an outside partner will be brought in to inject investment to the business.
At least seven ministerial aides have now put their names to a Commons motion opposing the controversial policy - with several indicating they might quit over the issue - bringing the total number of Labour rebels to 137.
But Mr Brown urged critics to face facts: "I think we all recognise that the Royal Mail, which is part of the fabric of our country, faces huge challenges.
He went on: "I do say to you that guaranteeing a £25 billion pension fund, spending £1.9 billion on the post office network, maintaining a universal service obligation can be achieved if we make the investment that is necessary to secure the Royal Mail for the future.
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"And that is what's behind our proposals to get someone to help us invest so that we can prepare the Royal Mail for the international and national challenges ahead."
