From April 2011, the time off can be taken during the second half of a baby's first year if the mother returns to the workplace.
The plans were outlined in September by Prime Minister Gordon Brown, who said they would give "couples more freedom, dads more rights and children more time with the two people who love them most".
At present fathers are entitled to two weeks' paid leave and mothers to 52, 39 of them paid. The government conceded take-up was likely to be low, with fewer than one in 16 fathers expected to leave work for a period of full-time childcare.
Ministers said the impact on small businesses would be minimal: fewer than 1 per cent are expected to be affected by the changes.
However, British Chambers of Commerce director general David Frost has written to Business Secretary Lord Mandelson demanding an end to the "constant threat of tinkering to employment law".
Mr Frost called for a three-year moratorium on changes while firms dealt with the economic downturn.