The Conservative party's split on Brexit continues to affect the process, former Treasury minister Lord Jim O'Neill has told the BBC.
Lord O'Neill, who left government a year ago, said much of the "rationale for Brexit" had been to deal with that division.
He also said the UK's negotiators needed to be "more realistic" in their talks with the European Union.
The government said it was "working together to deliver Brexit".
The UK was looking to achieve a new deep and special partnership with the European Union, a government spokesperson said.
"We've entered these negotiations with a clear position, and a spirit of pragmatism, determined to deliver an orderly exit that allows us to grasp the opportunities of life outside the EU," the spokesperson added.
The government has faced criticism this week from a number of pro-European Conservative MPs for its handling of a key piece of Brexit legislation.
One of the MPs, Anna Soubry, called on Thursday for the government to put all Brexit options "back on the table", although the Tory MPs are not expected to rebel against the bill at this stage.
Labour plans to vote against the EU (Withdrawal) Bill as it stands, but also faces the prospect of MPs rebelling against its leaders.
When asked how he would rate the government's Brexit process, Lord O'Neill told the Today Programme: "I think everybody has to, sadly, keep their mind on the fact that much of the whole rationale for Brexit was driven by politics and in particular the divisions inside the Conservative party.
"It looks to me, unfortunately, that one year on, that is still the case."
Describing himself as a "not particularly passionate Remainer", Lord O'Neill said he wished political splits between parties and within the Tories would no longer "dominate the policy discussion".
However, others could argue the complexity and timetable of Brexit, as well as specific disagreements over money and Northern Ireland are affecting the process more.
David Cameron was accused of promising the Brexit vote in 2013 to appease Tory Eurosceptics at a time when Ukip was gaining popularity.
But his advisers have said the referendum, held in June last year, was unavoidable because of divisions in the country on the issue.
Lord O'Neill was brought into government in 2015 after Mr Cameron won that year's general election.
He briefly stayed on under Mrs May after the Brexit vote, but left in September 2016.
Lord O'Neill, who is judging the Indigo economics prize, said the UK's Brexit negotiators also "need to be more realistic" about trying to keep advantages of EU membership.
He said: "The last thing the EU wants is for a big country to leave and there be no consequence, because at some point there would be others that might think: 'Well, if it's that easy, and you can get away from the worst bits but keep all the good bits, then I'll have some of that too.'
"That's the key issue that's driving the negotiations on the European side. It frequently seems that our negotiators don't seem to appreciate that."
The government has said the UK will leave EU trading arrangements, including the single market and customs union, after Brexit.
In their place, the UK will seek an ambitious free trade deal with the EU, while also being free to pursue trade deals with other countries, it has argued.
The European Commission's chief Brexit negotiator, Michel Barnier, said on Thursday the UK wanted the EU to suspend its single market and customs union on the Irish border.
He said Britain had chosen to leave, and so it was up to the country to come up with an acceptable plan for future relations between Northern Ireland and the Republic of Ireland.
The UK said both sides were "closely aligned" in what they wanted to achieve.