A £1,000 transferable marriage tax allowance for lower rate taxpayers will be introduced by the government from April 2015 – one month before the general election – David Cameron announced on Friday night.

On the eve of the Tory conference in Manchester, the prime minister said the government would recognise the "special" status of marriage by offering a tax break which goes further than the £750 proposed in the Tory manifesto in 2010.

The scheme will mean four million married couples and those in civil partnerships who are lower rate taxpayers will be able to transfer £1,000 of their personal tax allowance to their spouse or civil partner. The scheme will be worth up to £200 a year.

In a Daily Mail article, Cameron wrote of the importance of marriage when he said nothing he has achieved since his marriage in 1996 would have been possible without the support of his wife, Samantha.

He wrote: "There is something special about marriage: it's a declaration of commitment, responsibility and stability that helps to bind families. The values of marriage are give and take, support and sacrifice – values we need more of in this country. Of course this will be true if you're gay or straight – and in a civil partnership or a marriage. This summer I was proud to make equal marriage the law. Love is love, commitment is commitment."

The announcement came as the father of the Tory modernisation project threw his weight behind Cameron's plan, describing marriage as a glue in society that should be recognised in the tax system. In a Guardian interview ahead of the conference, which opens on Sunday in Manchester, the Cabinet Office minister Francis Maude said: "We want a strong society. Marriage is one of the institutions that creates glue in society and it makes sense to recognise that."

But Julianne Marriott of Don't Judge My Family said: "It's about promoting a fantasy 1950s family and won't go to many of the families who need support the most. In these tough times government should be helping families, not judging them."

The announcement by the PM has been the subject of intense negotiations within the coalition after the Liberal Democrats demanded the introduction of universal free school lunches for infants in return for agreeing to the timing of the marriage tax allowance proposal.

Nick Clegg infuriated No 10 when he contrasted the £600m-a-year cost of his scheme with the similar price tag for recognising marriage in the tax system.

Labour dismissed the Tory plan. Rachel Reeves, the shadow chief secretary to the Treasury, said: "David Cameron's so-called marriage tax break won't even help two-thirds of married couples, let alone millions of people who are separated, widowed or divorced. He's so out of touch he thinks people will get married for £3.85 a week."

The Tories will meet for their penultimate conference before the 2015 general election cheered by the return of economic growth and signs that their party is closing Labour's poll lead.

But there was some irritation on Friday night when Matthew d'Ancona, a journalist with strong links to the Cameron camp, reported in a new book serialised in the Daily Telegraph that Cameron has held informal discussions with Clegg about forming a second coalition in 2015.

A No 10 spokesman said: "There are not, nor have there been any plans or talks to form a new coalition post 2015. The prime minister is entirely focused on running the country."

Some Tories have been unnerved by the Labour conference after Ed Miliband appeared to tap into popular discontent with the "big six" energy companies by pledging to freeze household fuel bills for his first 20 months in office.

Maude was highly critical of Miliband's announcement, which he claimed had quickly unravelled. He said: "It could have been a speech written by Len McCluskey [the Unite general secretary]."

The Tory leadership is bracing itself for the "blond moment" on Monday when Boris Johnson, who has fuelled speculation about his leadership ambitions by saying how he is missing parliament, arrives in Manchester. In an FT interview, the mayor of London compared himself to the founder of the Roman empire and said he wished he had been in parliament for the vote on Syria.

Cameron unveiled the first conference announcement when he said the government would provide £400m for the Cancer Drugs Fund. The move will give thousands more patients access to drugs and boost the fund's project to sequence 100,000 genomes (individual DNA codes).