The last time Parliament was forced to decamp was during the Second World War when London was under threat from the Luftwaffe and bombs fell on Westminster. This time MPs may be forced out because of a leaking roof.

The parliamentary authorities are examining plans to move out of the Gothic palace for a £250 million building programme to replace the disintegrating iron roof and ageing wiring. MPs will spend the next three years discussing different proposals, which could result in the chamber of the Commons being relocated to the Queen Elizabeth II Conference Centre, a 1986 concrete monolith on the other side of Parliament Square.

Sources involved in the discussions are aware that MPs are likely to be hostile, but point out that a move out could cut the length of the work from 25 years to two or three. At the moment, building work is usually carried out during the summer recess, but this may not provide a long enough break for such a substantial overhaul.

The plan would affect the Commons, select committees and possibly the Lords, but MPs and their staff would be able to stay in their offices, which are mostly in outbuildings adjacent to the Palace of Westminster. Such a move would also allow for other, more radical changes, including a glass roof over one of the open courtyards and even turning the shooting gallery into a swimming pool.

According to the annual report of the House of Commons Commission: “In 2007-08 planning will be undertaken for a number of major work projects, namely the renewal of the mechanical and engineering equipment in the Palace and the refurbishment of the cast-iron roofs for the first time since the 1840s. These extensive projects are subject to detailed planning and business case approval.”

Four buildings are understood to be under consideration. They are:

— Church House, which opened in 1940 and was used by Parliament in 1940, 1941 and 1944. It was chosen by Winston Churchill because it survived a direct hit in 1940.

— Methodist Central Hall, which opened in 1912 on Parliament Square. In 1946 the inaugural meeting of the General Assembly of the United Nations took place here.

— The Queen Elizabeth II Conference Centre, opposite Westminster Abbey, a business and conference centre over seven floors, with concrete façade.

— County Hall, the Baroque building across the Thames from Parliament that opened in 1922 and was home to Greater London Council until it was abolished in 1986. A debating chamber with a capacity of 200 survives.

Middlesex Guildhall, a former Crown Court on Parliament Square, is not available because it is being refurbished for use as the new Supreme Court. The decision must be made by the House of Commons Commission, chaired by the Speaker, and parliamentary sources say that there is a 10 to 15 per cent chance of such a move happening. Building work would start within five years.

Parliament does not have a distinguished record organising big building programmes. The visitor centre, which was due to open in January last year, is still not finished and in July a project recovery manager was appointed. The commission has pledged to hold a “lessons learnt” inquiry into what went wrong and has approved the appointment of a new director-general of facilities and support.

Patrick Cormack, the Tory MP and staunch defender of Parliament, was astonished that MPs had discussed such a plan. “I would be utterly flabbergasted. It might be necessary to have certain committees somewhere else but you cannot decant the whole of Parliament. It is just impossible.”

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