Downing Street has insisted Britain will leave the customs union after Brexit amid claims of Tory disunity over the UK-EU future relationship.
Theresa May has faced calls to set out clearly what she wants to secure from the negotiations leading up to the UK's departure in March 2019.
But a No 10 source said "to put this to rest, we are categorically leaving".
It comes ahead of a week of key Brexit meetings with the EU chief negotiator and with her most senior ministers.
On Monday, the prime minister and Brexit Secretary David Davis will meet Michel Barnier ahead of the next round of negotiations getting under way.
Later, talks between officials will focus for the first time on the transitional period planned for after Brexit.
Potential sticking points include citizens' rights, with the UK insisting EU nationals arriving during this time should not have the same rights as those who arrived before Brexit day.
Mr Davis and Mr Barnier are expected to reveal the progress of negotiations on Friday, reports BBC.