The bookmaker said it had been hit by a delay in relaunching its website but still hoped to meet market expectations.
Chief executive Richard Glynn has made improving the company's digital offerings a feature of his strategy since taking over in April 2010.
He said: "As stated previously, we anticipated that the phasing of our investment programme, increased marketing expenditure, planned operational losses associated with new international licences and the withdrawal from certain international markets would result in a decline in digital profits year over year.
"This decline has however been exacerbated primarily by a poor sportsbook margin in Q2 and by delays in the delivery of tech-nology projects leading to lower-than-expected revenue in quarter two.
"As a result we now expect digital profits in the first half to be down further than anticipated at around half that delivered in 2011."
The slump in digital, which delivered £31 million of profit in 2011, has been offset by the firm's 2000 site retail business which is said to have outperformed the market.
Mr Glynn believes the £50m investment in digital will pay off and added: "We remain confident that on-going development of the digital business will enable us to grow digital profits in 2013 and beyond."
The company has also been affected by a lack of patriotic betting fever at the Euro 2012 football tournament with English fans less keen than in previous years to back their national team.
The progress of favourites such as Spain to the latter stages of the competition also hurt margins.
It had hoped to replicate its performance in the World Cup in 2010 when a high number of draws and low goal-scoring left punters clear losers.
Ladbrokes' plans for growth include expanding into new territories such as Denmark and Spain and turning around its struggling poker division.
It has also been heavily involved in marketing its sportsbook, casino and games offer.
Analysts recently said there were signs that its investment was starting to pay off as digital revenues grew 5.9% in the first quarter of 2012.
