LONDON - A parliamentary by-election announced Thursday is being seen as a big test for Britain's main opposition leader, Labor's Jeremy Corbyn.

One of his Members of Parliament, Heidi Alexander, has resigned after being appointed to work for London's Labor mayor Sadiq Khan as deputy mayor for transport in London.

The election will be held in the Lewisham East constituency, one of the London parliamentary seats, on June 14.

The snap general election last June Alexander kept her seat in the House of Commons with a comfortable majority of over 21,000. She won almost 70 percent of the votes, more than 20,000 ahead of her closest rival, the Conservative candidate.

Health in September 2015, but resigned from Labor's shadow cabinet in June 2016, one of the first front-bench politicians to quit Corbyn's shadow cabinet.

Election experts will be carefully watching to see what percentage of the vote Labor wins next month, particularly after last week's local council elections in which Labor failed to score a major assault on the governing Conservatives.

The resignation of Alexander led Thursday to the playing out of one of the most quirky traditions of British political life, dating back almost 400 years.

In 1624 a resolution passed by the House of Commons prohibited MPs directly from resigning their seats.

Death, disqualification and expulsion became the only means by which an MPs seat could be vacated during the lifetime of a parliament.

Ever since MPs wanting to resign has to go through the process of accepting an office of the Crown, which automatically disqualifies the MP from holding a seat in the House of Commons.

So at the Houses of Parliament today the Chancellor of the Exchequer, Philip Hammond, appointed Alexander to the unpaid job of Crown Steward and Bailiff of the Manor of Northstead.

The appointment immediately paved the way for her resignation from the House of Commons, and maintained one of the old traditions of political life in Britain.