The Liberal Democrats will announce plans to triple statutory paternity leave to six weeks on Wednesday, the third day of the formal general election campaign.

The plan will be unveiled by party leader Nick Clegg and Lib Dem equalities minister Jo Swinson, standing for re-election in East Dunbartonshire, coinciding with the introduction of shared parental leave across the country this week – a coalition policy the Liberal Democrats claim as their own.

The pledge, which will be in the party’s yet-to-be launched manifesto, will give fathers an extra four weeks off work with ordinary paternity pay (currently set at £138.18 per week) when they have a child.

Clegg is expected to say that for too long mothers have been told their place is at home with their child, while fathers are expected to return to work. “I want parents to choose for themselves how to balance work and family,” he will say.

Shared parental leave and pay comes into effect for babies due on or after 5 April or adoptions where the child is placed on or after that date. Under the new system, eligible couples will be able to share parental leave following the first two-week recovery period that mothers have to take off after birth, so up to 50 weeks’ leave and 37 weeks of pay can be shared.

The announcement comes the day before Nick Clegg will take part in a seven-way televised leaders’ debate on ITV. The deputy prime minister said it was “self-evident” he would not “triumph” at the event.

He told journalists travelling on the Liberal Democrat campaign bus that he had not prepared as thoroughly as he did for the debate with Gordon Brown and David Cameron ahead of the 2010 general election, which he was widely considered to have won.

“I’m not spending nearly as much time trying to think through every twist and turn. I’m not preparing nearly quite as methodically as last time,” he said. “The longer I’ve been in politics the more I think you can over-think these things. And it will be very different – there’ll be seven of us.”

He said that his televised debates with Ukip leader Nigel Farage taught him that “you might marshal the best arguments, but if there are people who play better to the gallery they are going to do well”.

“So I’ve been in politics long enough to know it’s not a beauty contest and it tends to be the case that newcomers or people not well known to the public tend to do best, so I enter with quite sober, realistic expectations. But I intend to enjoy it.”

The party’s plan to triple paternity leave would extend overall paternity leave to 58 weeks. The Liberal Democrats say they would consult on the policy with a view to implementing it at the earliest opportunity and by 2018-19 at the latest. The annual cost to the public purse has been estimated by the Department for Business Innovation and skills at £58m.

The letter states: “We run some of the leading businesses in the UK. We believe this Conservative-led Government has been good for business and has pursued policies which have supported investment and job creation.

“David Cameron and George Osborne’s flagship policy of progressively lowering Corporation Tax to 20 per cent has been very important in showing the UK is open for business. It has been a key part of their economic plan.”

It adds: “The result is that Britain grew faster than any other major economy last year and businesses like ours have created over 1.85million new jobs.

“We believe a change in course will threaten jobs and deter investment. This would send a negative message about Britain and put the recovery at risk.”

Other signatories, who have signed the letter in a personal capacity, include Lord Rose, the former head of Marks & Spencer, Sir Nigel Rudd, the chairman designate of engineering firm Meggitt, and Malcolm Walker, the founder of Iceland.

Labour on Monday launched its business manifesto in London but immediately faced criticism over a full-page advert the party had taken out in the Financial Times featuring quotes from a number of UK business leaders.

However, within hours four of the six company bosses had distanced themselves from the advertisement and Labour because of concerns they were being used for political ends.

Siemens UK, the electronics company, accused Labour of “overstepping the line” and said it had not been told that the quote would be used in a political advert.

Kellogg’s, the breakfast cereal company, said that “eyebrows had been raised” when the quote was used.

Mr Miliband has faced months of criticism from business figures after he criticised the tax status of Stefano Pessina, the Boots boss who warned that the Labour leader would be a “catastrophe” for Britain.

Mr Balls was also embarrassed earlier this year when he failed to name a single businessman that back Labour, instead referring to “Bill somebody”.

Meanwhile, George Osborne on Tuesday hailed official ONS figures which disclosed that GDP has increased by 2.8 per cent over 2014, up from forecasts of 2.6 per cent.

Average household incomes are now higher than they were in 2010, a milestone hailed by Mr Osborne as “good news for families and businesses across the country”.

The ONS also disclosed that household spending has grown by 0.6 per cent, or £1.6 billion, in the fourth quarter of last year, driven by a huge rise in people buying life insurance as the economy recovers.

Mr Osborne said: “We’ve got a hat-trick of good news about the British economy and with 37 days to go the election it’s another sign that changing course would put recovery at risk.”

He added: “So voters now face a stark choice: do we stick with a plan which is working, delivering growth and jobs, or do we put all that at risk with Ed Miliband whose policies of more spending, more borrowing and higher taxes will lead to economic chaos.”