Sir John Major has insisted his high-profile intervention in the Brexit debate is not intended to undermine Theresa May.
It comes after the former Conservative prime minister called for MPs to be given a free vote on the final Brexit deal.
Sir John has also called for there to be an option of putting it to the public in a second referendum.
In a speech in London on Wednesday, he urged Mrs May to stand up to the "ultra-Brexit" minority in her party.
It comes just two days before she sets out her vision for Britain after its withdrawal from the European Union.
When asked by ITV News Political Correspondent Romilly Weeks if there was a danger his intervention in the Brexit debate could undermine Mrs May, Sir John said: "I sincerely hope not".
He added: "That is certainly not my intention. But at the moment the whole of the advice being poured into the prime minister's ear seems to come from one section of the party and not the bulk party."
Sir John insisted he had set out things which he believes are important for the Conservative Party to consider "in the long-term".
"If we head as we are going at present there is a real danger that we will end up without an agreement and a hard Brexit.
"If that happens it will cause very great difficulties not only for our commerce and industry but for millions of people and it will hurt the people who have least, most."
Sir John added: "I think someone has to put those arguments at this stage before final decisions are taken on our negotiating strategy."
When asked what his thoughts were on the Prime Minister's position with the issue having no hard border in Northern Ireland Sir John agreed she is in an "incredibly difficult" position but said that "it isn't a new one".
"I remember going to Northern Ireland with Tony Blair warning of this during the referendum, at which point we were told by Brexit politicians that we didn't know what we were talking about on Ireland and there would be no problem. It is now apparent that there is a problem," he told ITV News.
But Sir John said that maintaining the Good Friday Agreement and all that has meant for Ireland is "very important" and that if joining a customs union is the only solution then "that is something we should do".
Sir John was asked if he agreed with Boris Johnson's proposal of using technology to avoid having a hard border put in place, to which he replied that the Foreign Secretary has a "partial solution" and he has "partially thought it through".
"You can probably have a technological solution for hard goods for cans of beans or bottles of beer, bottles of Guinness going from the south to the north, you can probably do that but when you're talking about animals, when you're talking of fresh food, you are going to have to have health checks on animals against BSE or diseases and the whole range of things of that sort and that is going to need a physical check.
"So you are going to need a border so there is some element of the trade that can probably be dealt with electronically but not all of it and that is the problem that nobody has yet been able to overcome," he added.