The new Guernsey-only service will start from 28 March next year, with Jersey yet to choose a ferry provider.
The cheapest journey the Guernsey Press could find for a return journey for two adults and two children in a car to Portsmouth from Guernsey on the traditional ferry in April was £310.
An April trip to St Malo under the same conditions was
£290.
The booking website went live at 5pm last night.
By 5.30pm yesterday, prices for the school summer holidays were already going up.
Still using a car, with two adults and two children at the start of the summer holidays saw a return UK trip starting from £410 and return trip to St Malo starting at £370.