The starting gun has been fired for three by-elections likely to be viewed as an important test for the main parties.

Campaigning for the June 29 began after writs were moved for polls at Blaenau Gwent in South Wales, caused by the death of independent MP Peter Law, and Bromley and Chislehurst in south east London, previously held by Conservative MP Eric Forth.

An election for Law's seat in the Welsh assembly has already been called for the same day.

Labour will hope to regain Blaenau Gwent, a party stronghold for more than 80 years until Law defected from the party in protest at the imposition of all-women shortlists.

Law's former agent Dai Davies will contest the Westminster seat as an independent and his widow Trish Law the assembly seat.

Labour lost its majority in the assembly with Law's defection, and victory in the by-election would restore it - although opinion polls have suggested they could lose again.

The race for Bromley and Chislehurst, held for the Tories by Forth with a majority over 13,000 in 2005, is the first by-election test of David Cameron's leadership.

Cameron will hope to capitalise on the gains of May's local elections, while Liberal Democrat leader Sir Menzies Campbell will want to banish criticisms of his performance by repeating the party's unexpected success in the Dunfermline and West Fife by-election in February.

The Lib Dems, then run by Sir Menzies as acting leader, managed to overturn an 11,500 Labour majority.