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.

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.

A spokesperson for The Metropolitan Police said: “At 10.25hrs on Wednesday, 3 September a mobile phone was found during a routine search of the House of Commons chamber by Met officers.

“Enquiries have led officers to believe that the phone was purposely placed in a location with the aim of causing disruption to business in the House.

“An investigation has been launched and enquiries are ongoing. We have updated the Speaker and Clerk of the House.”

A UK Parliament spokesperson said: “Parliament is a public building and we facilitate the visits of thousands of people to the estate each week.

“The safety and security of everyone who works or visits here remains our top priority, and we have robust and proactive security measures in place – this includes ensuring that visitors and their belongings are security screened, along with monitoring and routine searches of areas that are open to the public.

“Whilst we cannot comment on the detail of our processes, we can confirm that a mobile phone was removed from the Commons Chamber on 3 September – demonstrating the effectiveness of the security measures we have in operation.”

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.”