Google, Microsoft and Yahoo have been fined a total of $31.5m by the US government for advertising illegal gambling websites.

Microsoft will pay $21m, Yahoo will pay $7.5m and Google will pay $3m in a settlement agreed with the US Department of Justice (DoJ).

Microsoft's fine is the largest as its case dates back 10 years. The other two search sites have run gambling and sports betting adverts for a lesser period.

Charges by DoJ in the case claimed that the three companies had violated the Federal Wire Wager Act, related tax laws and the individual gambling regulations set up by individual US states.

The settlement follows 18 months of negotiations between the internet companies and the DoJ.

Microsoft's fine includes a $7.5m contribution to the International Center for Missing and Exploited Children and an agreement to run a three-year $9m advertising campaign warning young people of the dangers of gambling.