The Rev Ian Paisley has asked Gordon Brown for reassurance that British government plans for electronic borders will not see Northern Ireland citizens required to produce passports in order to travel to other parts of the United Kingdom.
The DUP leader wrote to the British prime minister following recent speculation about the effects of the new UK Borders Bill and Taoiseach Bertie Ahern's first comments on the Irish Government's necessary response to London's drive toward e-border controls.
While he and his party would support "the need for effective immigration controls across the United Kingdom", the DUP leader told Mr Brown that "it would be very dangerous if, as a result of improving controls for Great Britain, Northern Ireland was to be treated differently to the rest of the United Kingdom".
Dr Paisley also told Mr Brown the UK's land border with the Republic would "have to be taken into account" in ongoing discussions between the two governments.
Dr Paisley wrote to the prime minister in his capacity as Democratic Unionist Party leader rather than as Northern Ireland's First Minister in a matter that would not lend itself to "joint" action with Sinn Féin Deputy First Minister Martin McGuinness.
He told Mr Brown: "Obviously the common travel area between the UK and the Republic of Ireland means it is possible for people to enter the Republic and then travel to the United Kingdom, either directly to Great Britain from the Republic or into Northern Ireland and then to other parts of the UK."
Dr Paisley then indicated his opposition to any suggestion that new border controls might operate as between Britain and 'Ireland'.
"I would ask for reassurance from you that the [ British] government will not bring forward any legislation which would require citizens from one part of the United Kingdom to present a passport in order to travel to another part of the UK," he told Mr Brown
And he went on: "For obvious reasons the government does not believe that border controls are necessary when entering the UK across the land border with Northern Ireland but if the government is to tackle immigration on a national basis then the UK's land border with the Republic will have to be taken into account."
Dr Paisley concluded by observing that Mr Brown had "rightly pointed out the virtues of retaining the union and of promoting a sense of Britishness"
He declared: "British citizens living in Northern Ireland should therefore not have to find themselves being treated differently to their countrymen living within the rest of the UK."