He urged decision-makers to take a tough-line approach with European "bullies", citing the former prime minister of the "Iron Lady", Margaret Thatcher.
Writing in The Sun, he said: "We can really forge an exciting, strong and prosperous future outside the EU. However, at this time, government negotiators have no confidence in themselves.
The UK has surrendered to EU demands for Britain's commitment to defense, intelligence and security, as well as complying with the rules of the customs union and the single market, he added.
And in one more stroke, the United Kingdom agreed to deliver £ 39 billion when Europe asked, he said.
According to Chancellor Philip Hammond, the UK is required to pay at least £ 36 billion of the exit bill, even if we do not seal an agreement because the UK is unlikely to win a legal battle for cash.
Hammond told the Cabinet in mid-October that the Treasury said it would be almost impossible to keep the money pledged to the EU for divorce.
However, one of the major concerns of Mr. Duncan-Smith is the agreement of the United Kingdom that if we do not get a trade agreement with the EU, Northern Ireland will remain in the customs union.
He added: "We finally made perhaps the biggest mistake of all; we agreed that if we did not get a trade agreement with the EU, Northern Ireland would remain in the EU Customs Union and in some parts of the single market.
"In effect, we were giving power to the EU to break the union of the United Kingdom, we gave it away.
"It's time to put the support in the north of Ireland and defend British democracy and freedom or we will leave and they will not get their money, their security, intelligence or defense aid."
Support is in place in the event that a Brexit agreement is not achieved, however, Europe has interpreted this to mean that Northern Ireland will continue to follow its trade rules to avoid a hard border.