New Northern Ireland Secretary Peter Mandelson was advised to avoid referring to the Republic of Ireland as “The Free State” or “The Republican Government” when he took up the role in October 1999, newly declassified files have revealed.

Mr Mandelson had recently replaced Mo Mowlam as Secretary of State for the region when a first day briefing paper was provided for him by the Northern Ireland Office.

The 164-page document contained guidance information for Mr Mandelson and other new NIO ministers on the state of politics in Northern Ireland at a time when preparations were advancing towards devolving power to the new Stormont Assembly which had been established by the 1998 Good Friday Agreement.

The briefing paper contained extensive information on the state of the Northern Ireland political parties, their key figures, the security situation, media organisations, and relations with the Republic of Ireland.

It stated: “In recent years the working relationship between the two Governments has been particularly strong and effective.

“The tone has been set by the close personal relationship between the Prime Minister and the Taoiseach, Bertie Ahern.”

The briefing paper included a section titled “Terminology, including expressions best avoided”.

The new Northern Ireland Secretary was advised to avoid referring to Great Britain as “the mainland”.

The paper stated: “A term used by unionists but not generally acceptable. Reference should be made to GB.”

The document said the term “Northern Ireland is preferable to Ulster or the Province”.

It added that “the North” is “an acceptable colloquialism used by both communities”.

Mr Mandelson was told the term “the Province” “is best avoided when talking to nationalists”.

'The Republican Government'... to be avoided at all costs, as being deeply offensive to that Government

He was also told when in Northern Ireland to avoid the phrase “Being out here – because it smacks of the colonial outpost attitude”.

The new Northern Ireland Secretary was provided with a list of terms to avoid using when referring to Ireland and the Dublin Government.

The document stated: “The Free State – official name of the Republic from 1922-37. Sometimes used as a derogatory term by unionists and republicans. Unacceptable.

“Eire – official name of the Republic in Irish. Refers to the island of Ireland and implies claim to the 6 counties comprising Northern Ireland. Unacceptable.

“The Republican Government – A term occasionally used to describe the Government of the Republic of Ireland. To be avoided at all costs, as being deeply offensive to that Government.”

The briefing paper also advised against using the term “Prods” to refer to Protestants, and “Fenians” or “Taigs” to refer to Catholics.