Who were the Social Democratic Party?

Commentators have been comparing the resignations of seven MPs from the Labour Party on Monday to the formation of the Social Democratic Party (SDP).

But why has the latest change in Westminster's political layout reminded some of events dating back almost 40 years?

• Seven MPs leave Labour in Corbyn protest

• Gang of Four on Jeremy Corbyn

• The legacy of the SDP's Gang of Four

In January 1981, four former cabinet ministers, Bill Rodgers, Shirley Williams, Roy Jenkins and David Owen, announced their intention, following the party's Wembley conference earlier that month, to move away from Labour.

The "gang of four", as they were nicknamed, issued what became known as the Limehouse Declaration from Mr Owen's house in east London.

Unhappy with the direction Labour was moving in - namely, to the left - they claimed "a handful of trade union leaders [could] now dictate the choice of a future prime minister".

"The gang of four" - Bill Rodgers, Shirley Williams, Roy Jenkins, David Owen

The founders wanted "a new start in British politics" and proposed a Council for Social Democracy to "rally all those who are committed to the values, principles and policies of social democracy".

The leader of Labour at the time - Michael Foot - said he wanted them to stay and help to shape the party.

But two months later, that council became the Social Democratic Party and eventually 28 Labour MPs would join the ranks, as well as one Conservative.

In June of the same year, the SDP joined in an electoral alliance with the Liberal Party to take its "new politics" to the polls.

The alliance elections

Mrs Williams was the first member to stand as an SDP candidate and win, taking the seat of Crosby in November 1981.

And come the election in 1983, the alliance was shown to have growing support - securing 25% of the vote.

But thanks to the "first past the post" voting system in British elections, this amounted to only 23 MPs.

After the election, however, Mr Foot, resigning as leader, blamed the alliance for siphoning off Labour votes and giving Margaret Thatcher and the Tories another term in government.

The alliance went on to fight another election, in 1987, but again failed to make much of an impact on the numbers in the Commons - with almost 23% of the vote amounting to just 22 MPs.

It was decided in 1988 that the SDP and the Liberal Party should merge - and the Liberal Democrats were born in October 1989.

Mr Owen was unhappy with the decision and led a much smaller version of the SDP until 1990. Subsequent incarnations have not managed to make an impact on Westminster elections.