Network Rail have confirmed that 1,000m of cable has been stolen from the line between Bedford and Bletchley.
A spokesman told ITV Anglia that rail delays would continue for the rest of the day while engineers worked to make repairs.
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Dry with clear or sunny spells. Winds easing overnight.
Very warm with sunshine. Chance of thundery showers late morning.
A hot, humid and sunny start before cloud increases as a narrow band of thunderstorms spreads northeast. These clear during the afternoon.