Previously confidential files from 1983 released on Thursday by the National Archives in Kew shed new light on the ongoing attempts by Prime Minister Margaret Thatcher to deal with the political and security situations in Northern Ireland and, in particular, the threat by Sinn Féin to overtake the SDLP as the voice of Northern nationalism.

Sinn Féin's record 13.4% of the regional vote in the June 1983 election and the return of its President, Gerry Adams, as MP for West Belfast came as a shattering blow to Mrs Thatcher, who had returned to power with a renewed mandate after the Falklands war.

Ministers believed that up to a quarter of the Sinn Féin vote was down to impersonation and intimidation.

At a cabinet meeting in June that year, Northern Ireland Secretary Jim Prior warned colleagues that the republicans' success could lead to the destruction of John Hume's SDLP....