By John Campbell BBC News NI Economics & Business Editor
Theresa's May former chief of staff says Boris Johnson's Brexit deal gives the EU what it originally wanted on Northern Ireland.
Lord Barwell was one of Mrs May's closest advisors when she was negotiating with the EU.
He said Mr Johnson's deal is "in large part" Mrs May's deal, but is mostly worse in regard to Northern Ireland.
"The government has gone back to what the EU originally wanted, a Northern Ireland-only arrangement," he said.
"The result is that goods will have to undergo customs checks when they are moved from Great Britain to Northern Ireland, creating a border in our single market."
He told the House of Lords that would be bad for the union and bad for the economy.
However, he conceded that the consent mechanism for Stormont is an improvement on the deal that Mrs May had negotiated.
Under Mr Johnson's deal, Northern Ireland will effectively stay in the EU's single market for goods but Stormont can vote to end that arrangement.
Mrs May's deal could have seen Northern Ireland enter a "backstop" arrangement which could only have been ended with the EU's approval.
Lord Barwell also urged the government to be honest about how difficult the next stage of the Brexit process will be.
The prime minister hopes to negotiate a trade deal with the EU before the end of this year.
Lord Barwell said that even if the UK wanted to stay aligned with EU rules the negotiation would not be easy.
"We are about to negotiate something completely unprecedented.
"A free trade agreement which is not about removing barriers to trade but agreeing when and to what extent they will have to be put up."