A negotiator of the Good Friday Agreement has accused Boris Johnson of "playing games" with the peace process.
Jonathan Powell was the government's chief negotiator for Northern Ireland from 1997 until 2007.
Unionists have expressed concerns about Mr Johnson's Brexit deal splitting NI from the rest of the UK.
Mr Powell said how the prime minister is dealing with Northern Ireland right now was coming from a "position of ignorance".
Mr Johnson agreed a new deal with the EU last week, which included special arrangements for Northern Ireland.
It would allow the UK as a whole to leave the customs union, but Northern Ireland would continue to follow some EU customs rules.
The Northern Ireland Assembly, if sitting, would get a straight majority vote in 2024, to decide whether to keep following the rules.
The DUP has opposed the deal. It had argued that any vote must be on a cross-community basis, in line with how most contentious matters are decided at Stormont.
That cross-community voting system was set up under the Good Friday Agreement in 1998, which Mr Powell helped negotiate.
He told BBC's The View: "It's so unwise to play games with this. This peace process is a seesaw, if you jump on one end, the other end's going to go flying in the air.
"A sensible government should be balancing that."
He said the new Brexit deal undermines the identity of unionists in Northern Ireland, and warned against people taking an "I-told-you-so" attitude to the DUP for trusting Boris Johnson.
Mr Powell added: "I can understand why people feel glad the DUP has been shafted after them making this close alliance.
"But that's an unwise attitude for the long term. This peace process has two parties at the centre, and if either of their identities are attacked we'll have difficulties sustaining that."
The former negotiator said Mr Johnson had not understood what he had agreed to for Northern Ireland in his Brexit deal.
He added that hardly anyone in the government had any "real knowledge" of the situation.
Asked about the ongoing political deadlock in Northern Ireland, Mr Powell described it as very frustrating.
He said that as long as Brexit is "hanging over people, it will be very hard to get the institutions back up and running, because it undermines the Good Friday Agreement and the peace process".
"It doesn't have to be terminal if people come to their senses - it can be saved but it is very worrying."
This interview will be broadcast on The View on BBC One NI on Thursday night at 22:35 BST.