The supermarket ran ads in the national press to highlight the new price, under the headline 'Britain's cheapest milk?'.
Morrisons' move comes after Tesco raised the price of four pints of standard from £1.25 to £1.49 on 31 May, followed by price increases in Sainsbury's and Waitrose.
Following Tesco's increase to £1.49, Asda still selling at £1.25 as The Grocer went to press ran an ad highlighting the price difference between its milk price and Tesco's. However, Tesco continues to sell four pints for £1 under its tertiary Creamfields brand.
Morrisons said its 50p promotion was for a limited time only, and it was not paying its farmers any less as a result.