The decision to recall parliament during recess is one only taken during times of national or international crisis or when there is a threat to UK security.
It is a rare event and the decision to bring MPs back early from their summer recess on Friday in the wake of the terrorism attack on the United States indicates just how grave the government believes the situation to be.
Here are the times that parliament has been called out of recess since World War Two:
In September 1949 an economic crisis prompted the devaluation of sterling to the tune of 30.5%, and a reduction in spending and investment expenditure by £250m.
Britain's involvement in the Korean war prompted parliament's recall in September 1950 because of the extreme pressure that was being put on national finances.
Just more than a year later, in October 1951, parliament was recalled in order for it to be dissolved ahead of a general election.
In September 1956 the Suez Crisis, prompted by Egypt seizing and nationalising the Suez canal saw MPs returning early from the summer recess.
As in 1951, September 1959 saw MPs recalled to tie up the loose ends ahead of dissolution and a general election.
The building of the Berlin Wall in 1961 prompted a massive international crisis and meant that parliament was recalled in October.
In January 1968 MPs returned so that the then prime minister Harold Wilson could make a statement on government cuts in expenditure.
Later that year, in August, the Soviet invasion of Czechoslovakia prompted parliament to be reconvened during the summer recess. On that occasion the Biafra war in Nigeria was also discussed.
In May 1970 parliament was recalled again to be dissolved ahead of a general election.
The introduction of internment without trial in Northern Ireland by the Stormont government and violence in the province saw parliament reassemble in September 1971 to debate the crisis.
In 1974 economic difficulties in the wake of the 1973 oil crisis and related industrial action saw parliament recalled in January.
The collapse of the power-sharing executive in Northern Ireland in June 1974 after just three months saw MPs back in the Commons debating on the future of the province.
The next time parliament was recalled was in the wake of the Argentine invasion of the Falkland Islands in April 1982, so that the British reaction could be discussed.
In September 1990 the Gulf crisis, prompted by the Iraqi invasion of Kuwait, prompted parliament's recall.
A financial crisis in the wake of the UK's exit from the European exchange rate mechanism meant parliament returned for an emergency statement in September 1992.
The growing crisis in Bosnia in May 1995 led to emergency debates being held during the Whitsun recess on the government's decision to commit 6,700 more troops to Bosnia.
The atrocity in Omagh when dissident republican terrorists bombed the town prompted parliament's recall in September 1998 so that emergency legislation could be enacted.