An investigation has been launched after security guards foiled a plot to embarrass the prime minister by playing “sex noises” from a phone hidden in the Commons.

Parliamentary staff carrying out a routine sweep of the chamber ahead of PMQs uncovered the mobile phone, which had been taped to the underside of the Commons table, Sky News understands.

It appears that the device had been set to play a sexually explicit recording if rung, and that the person behind the plot hoped to make this happen as Sir Keir Starmer faced off against Kemi Badenoch on Wednesday lunchtime.

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But, a Commons source has told Sky News, that parliamentary security guards found the device before it could go off.

The source explained that the phone had been attached to the underside of the table, near the front bench, with double-sided sticky tape, and that this had lost its grip, leading to the phone lying exposed on the floor.

The phone later rang twice during PMQs with a “sex noise” ringtone, but it had already been removed from the chamber.

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Angela Rayner’s tax arrangements and the government’s level of borrowing dominate the first PMQs after the summer break.

However, while the plot – believed to be a prank – was avoided, security staff do not know who was behind it or how the phone came to be there.

It is being treated as a serious security breach, and it is understood that there is no clear footage of the phone being planted.

Hundreds of parliamentary security staff were on strike over pay and conditions on Wednesday morning, meaning visitors were banned from the parliamentary estate.

It is not the first time there have been breaches of parliamentary security in recent years. A group of semi-naked men and women glued themselves to the glass of the public gallery in the Commons in 2019, to protest about climate change. A dozen people were subsequently arrested.

Responding to a request for comment, a parliamentary spokesperson said: “The safety and security of all those who work in Parliament is our top priority, however we cannot comment on specific security measures.”