Theresa May has insisted that the UK must leave the customs union after Brexit in order to strike trade deals around the world.
The Prime Minister is facing a series of parliamentary battles in the coming weeks over the UK's future customs arrangements with the EU.
After a massive defeat on the issue in the House of Lords, pro-European Tory MPs are set to voice their concerns over withdrawal from the customs union on Thursday.
But speaking on a local election campaign visit, Mrs May insisted that should would not change course.
She said arrangements which are as "frictionless as possible" with the EU and the ability to strike trade deals around the world were both achievable under the proposals set out by the Government.
On a visit to a firm in the West Midlands she told the BBC: "Coming out of the customs union means that we will be free to have those deals, deals that suit the UK.
"But I also recognise the importance to businesses like this of being able to have as frictionless a border as possible into the European Union."
Downing Street insisted the Government's position had not changed since Mrs May delivered her Mansion House speech in March.
Those plans included two options, either a "customs partnership" effectively collecting duties for Brussels for goods arriving in the UK but intended for EU markets or a "highly streamlined" arrangement making use of technology and regulatory co-operation.
But Mrs May is set to face calls from leading Brexiteers Boris Johnson, David Davis and Liam Fox to abandon her preferred form of customs deal, according to The Times.
A showdown is expected to come at a meeting of the Cabinet Brexit committee scheduled for Wednesday when the trio will tell the PM that the "customs partnership" would be unworkable, the report said.
The Prime Minister's official spokesman said: "We have put forward two options. They were set out in the Mansion House speech.
"Those are still the two options we are moving forward with."
He added: "We think both of the two options which we have put forward can provide solutions to the Irish border and to having a smooth customs arrangement with the EU."
Mrs May will be given an indication of the scale of the opposition she faces from pro-EU Tories over her customs union plan in a Commons debate on Thursday.
Though the looming Commons vote on a pro-customs union motion would be a symbolic, non-binding one, it has the potential to deepen Tory wounds on Brexit.
The Prime Minister's spokesman played down its significance, saying Thursday's event was a "routine backbench business debate".
But binding Commons votes on a customs union during debates on the Taxation (Cross-border Trade) Bill and European Union (Withdrawal) Bill will prove harder for Downing Street to dismiss.
Downing Street rejected BBC reports that the issue could be treated as a motion of confidence in Mrs May's Government.
"That report was an absolute mystery to me and I note that it quickly disappeared from the bulletins," the spokesman said.
Many Remain-supporting MPs believe that keeping a customs union with the EU would limit the economic difficulties caused by Brexit and would help provide a solution to the Irish border issue.
Tory former chancellor Ken Clarke told BBC Radio 4's Today: "We won't make much progress until the Irish problem is solved."
But Eurosceptic former Cabinet minister John Redwood told Today the customs union was a "protection racket" and it was "extremely good news" that the Prime Minister was not softening her position.
Communities Secretary Sajid Javid tweeted that the referendum gave "clear instructions" to leave the customs union and accused some of seeing it as a "kind of post-Brexit comfort blanket".
The Housing and Communities Secretary's comments were rebuked by the CBI president Paul Drechsler, who tweeted in reply: "An MP of your talent should rise above ideology and lead based of fact, analysis and evidence - all of which favours a customs union. Always happy to discuss."