Breastfeeding rates were under 30% in the three areas trialled but the offer of vouchers of up to £200 led to 37 more mothers (of 108 eligible) continuing to breastfeed at six-to-eight weeks.
There will now be a much bigger trial of the Nourishing Start for Health (Nosh) scheme involving 4,000 women although critics, including leading doctors, say the scheme amounts to "bribery".
Official health guidelines recommend that babies are breastfed up to six months, to help protect infants from infections.
'A bribe for breastfeeding'
Dr Colin Michie, from the Royal College of Paediatrics and Child Health, said: "You're asking for a payback from some mums who can't necessarily give it. Not all mothers can breastfeed for a range of reasons - so it's just not fair. "It is essentially a bribe if you're offering money for breastfeeding. This pilot scheme has been a success - but it would be nice if we could incentivise mums to breastfeed without paying them."
Of the mothers eligible for the scheme, 58 initially signed up for the vouchers which were to be given out at five different time-points. Results for the three- and six-month stages are still being collected. Across England, 51% of women are breastfeeding by the six-to-eight week stage. Rates are much lower in poorer areas - sometimes reaching just 12%.
Dr Clare Relton, from Sheffield University's School of Health And Related Research public health section, is running the scheme, as part of a four-year research project.
She said: "The UK has one of the worst breastfeeding rates in the world - yet it gives better health outcomes to mums and babies, and saves the NHS money. During the last year, our small-scale tests have shown that midwives and health visitors were happy to tell mums about the scheme.
"We think this idea has the potential to increase breastfeeding rates in the UK, but we don't have enough information yet. So we are conducting a large-scale trial to help us find out how acceptable and effective the scheme is - and whether it would a good use of public money in the future."