Millions will vote in parliamentary elections in Scotland and Wales, and local elections in England.
The former US ambassador was given a security clearance despite concerns being raised during checks.
Since the 1990s, devolved governments across the UK have worked alongside the Westminster Parliament.
Sir Olly Robbins has defended his actions amid a row over the vetting of the former US ambassador.
Drawing a line under Lord Mandelson's appointment is proving impossible for the prime minister.
Sir Olly Robbins is expected to defend his decision not to tell Sir Keir Starmer that Lord Mandelson failed vetting.
The prime minister has been back in the Commons after it emerged the Labour peer failed vetting.
Mandelson vetting row shows Westminster continues to have the power to surprise, writes Chris Mason.
The prime minister says he only learned of security concerns around the ex-US ambassador earlier this week.
It is the first time the prime minister has been north of the border since the Scottish Labour leader Anas Sarwar called for him to step down in February.
The decision to send Peter Mandelson to Washington has become a scandal that just won't go away for Labour.
The decision to send Peter Mandelson to Washington has become a scandal that just won't go away for Labour.
The decision to send Peter Mandelson to Washington has become a scandal that just won't go away for Labour.
The PM is facing calls to resign over the revelation that Lord Mandelson did not pass security checks.
Sir Keir and the top official at the Foreign Office at the time are expected to face questions from MPs next week.
It's emerged that Lord Mandelson did not pass inital security vetting checks ahead of taking up the role of ambassador to the United States.
The Science, Innovation and Technology Secretary this week unveiled a £500m fund to boost British AI firms.
The former US ambassador was given a security clearance despite concerns being raised during checks.
BBC West Midlands political editor Elizabeth Glinka explains the significance of 14 local elections.
Ministers are considering restricting spot-on treatments and collars to prescriptions from vets and medical professionals.