Sir John Major has said he is "concerned" about the impact the deal between the Democratic Unionist Party and Conservatives could have on the Northern Ireland peace process.
The former prime minister, who began work engaging with the IRA to end the Northern Ireland conflict, said the peace process was still "fragile" and cautioned the pact could mean the Government will no longer be seen as impartial.
He told BBC Radio 4's World At One Programme: "People regard the peace process which was very hard earned over very many years by a lot of people, people shouldn't regard it as a given, it isn't certain, it is under stress, it is fragile.
"Although I don't expect it suddenly to collapse, because there's a broad consensus that wishes it to continue, I think we have to take care with it and take care that everything we do does not exaggerate the underlying differences that still are there in the Northern Ireland community."
He said he was "concerned" about the deal between the two parties, and saying he was "wary" and "dubious" about it "both for peace process reasons but also for others reasons as well".
Sir John said he wanted Theresa May to "succeed" and "stay" as Prime Minister, and that he understood and sympathised with her wanting to "shore up her parliamentary position" but said his "main concern" was the peace process.
"A fundamental part of that peace process is that the UK government needs to be impartial between all the competing interests in Northern Ireland."
He added: "The danger is that however much any government tries they will not be seen to be impartial if they are locked into a parliamentary deal at Westminster with one of the Northern Ireland parties, and you never know in what unpredictable way events will turn out, and we cannot know if that impartiality is going to be crucial at some stage in the future."
Sir John said it was "very important" that there was an "honest broker", stating that the "only honest broker can be the UK government".
"The question arises, if they cease to be seen as such by part of the community in Northern Ireland, then one can't be quite certain how events will unwind and that worries me a great deal about the peace process."
He said he could see problems "getting the Northern Ireland executive together", and expressed concerns over "the reintroduction of anything that remotely resembled a hard border", saying such a move would be "catastrophic" for the peace process and the relationship between Northern Ireland and Ireland.
"I simply think you need to be very wary of what could happen and therefore be very cautious about what you do, so that does concern me quite apart from my other concerns about an agreement with the DUP."
Sir John also said that events in the Northern Ireland tended not to unwind as expected, adding: "Here, with the peace process, we need to be prepared for the unexpected, we need to hope for the best but prepare for the worst.
"The last thing anybody wishes to see is one or other of the communities so aggrieved that the hardmen, who are still there lurking in the corners of the communities, decide that they wish to return to some form of violence.
"We really need to do everything we conceivably can to make sure that that doesn't happen, and that does require an impartial UK government."