It comes after similar letter by Church of England focused on inequality

Bishop of Shrewsbury defended advice, saying it is question of morals

Bishop of Shrewsbury Mark Davies: Bishops should 'highlight moral questions which underpin debate'

Roman Catholic bishops will today urge 4million worshippers to vote for political candidates who promote marriage.

Cardinal Vincent Nichols will say that stable families and help for the frail and elderly are key planks in advice about the general election for the church’s five million British followers.

The Right Reverend Mark Davies, Bishop of Shrewsbury, said yesterday: ‘No-one expects bishops to offer a political programme at election time but to highlight the moral questions which underpin the public debate.

'Faced with the unprecedented scale of family breakdown and its immense human and social costs, this election debate surely cannot ignore support for marriage.

'At the very foundation of society is respect for the sanctity of human life. We mustn’t forget the newly-elected parliament in 2015 will decide the legality of assisted suicide in our hospitals and care homes with far-reaching consequences for the way society regards the sick and the elderly.'

It contrasts with the widely-criticised manifesto by Anglican bishops last week which made no mention of marriage.

Instead the Church of England said families could best by helped by outside organisations like housing associations or credit unions.

Nor did the CofE bishops choose to remind voters of the difficulties of elderly people needing care nor the scourges of drugs and pornography.

The Church of England election letter was the first attempt to sway voters by the Anglican bishops and was widely criticised for its focus on inequality and the alleged evils of the market economy, consumerism and the legacy of Margaret Thatcher.

Roman Catholic bishops, however, have regularly given advice to churchgoers on the central issues they should consider in advance of each general election.

The Catholic advice, to be launched by Cardinal Vincent Nichols, has been prepared over three months by a team of four bishops and will strongly remind voters of the need to help stabilise families.

It comes at a time when family campaigners and some Tories have been calling loudly for bigger tax breaks to reward couples who marry, and an end to the tax and benefit penalties piled on mothers who choose to stay at home to raise their children.

Faith: The proposals will be launched by Vincent Nichols, pictured centre with newly-ordained priests in 2011

A bill to legalise assisted suicide is currently being considered in the Lords.

The bill, put forward by former Labour Lord Chancellor Lord Falconer, would allow terminally ill patients to be prescribed poison if two doctors approve.

A small tax break for married people was introduced last week in line with a promise made by David Cameron in the 2010 Tory manifesto.

The concession means married couples and civil partners who are not higher rate taxpayers will be able to transfer £1,000 of their tax free allowance to their spouse or partner.

It will adde £3.85 a week to the incomes of 1.6 million people, but analysts believe that in practice the gain will be even less for more than a million people, because for them Coalition benefit reforms will take away £2.50 of the gain.