Sir Menzies is making a speech to mark 100 days as leader

The radical proposal is to be unveiled by Sir Menzies Campbell in a speech to mark his 100 days as Lib Dem leader.

He is writing to local branch chairmen saying their choice of candidates will be a "significant factor" in determining central support.

Insiders describe it as a "carrot not a stick approach" to boosting diversity.

There is concern across the main parties at Westminster that the UK's overwhelmingly white, male Parliament is unrepresentative of the nation as a whole.

Conservative leader David Cameron has brought in an "A-list" of recommended candidates. Of the first 100 on the list, 10% are from ethnic minorities and more than half are women.

Trouble in Wales

Local Tories in at least 200 of the most winnable seats will be expected to pick their candidates from the list.

But the idea has come under fire in some quarters of the party from people who say they who oppose "positive discrimination" and those who claim their freedom of choice over choosing candidates is being eroded.

Labour led the way on boosting the number of female MPs with its women-only shortlists but that too has proved controversial.

At last year's general election the late Peter Law overturned a 19,000 Labour majority in Blaenau Gwent after running as an independent.