The King has hosted Bahrain’s sovereign at Windsor Castle amid calls from human rights campaigners for the release of political prisoners in the Gulf state.

King Hamad bin Isa Al Khalifa’s visit on Monday afternoon marked his silver jubilee year and included some time in St George’s Chapel to pay his respects to the late Queen Elizabeth II.

He arrived with his two sons, Sheikh Nasser and Sheikh Khalid, at the castle’s Quadrangle in a horse-drawn carriage as the Regimental Band of the Welsh Guards played the Bahraini national anthem.

The King greeted his fellow monarch and they inspected the Guard of Honour before returning to the dais to watch a military march.

King Hamad then joined Charles for tea inside the castle.

The visit sparked calls from activists at the Bahrain Institute for Rights and Democracy for the King to raise “pressing human rights concerns” with his guest over the imprisonment of political opponents in Bahrain.

Sayed Ahmed Alwadaei, the institute’s advocacy director, said: “Inviting Bahrain’s dictator is a grave error in judgment, as his victims continue to languish unjustly behind bars in Bahraini prisons.

“Disregarding human rights only emboldens further abuses and will send a clear signal that impunity prevails.

“We urge King Charles to raise the issue of political prisoners with King Hamad.”

A letter from the institute to the King mentioned several prisoners, including the leader of the dissolved opposition party Al-Wefaq, Sheikh Ali Salman, and Danish-Bahraini human rights defender Abdulhadi Al-Khawaja, whom campaigners claim have been “arbitrarily detained” for many years.

The letter read: “There are pressing challenges in Bahrain, including the arbitrary imprisonment of opposition leaders and activists for their role in calling for democratic change — calls that were met with violence and repression.

“Many of those advocating for reform have been subjected to torture, unfair trials, and life sentences.

“King Hamad’s visit to the UK occurs as families of death row inmates and political prisoners voice deep concerns over the fate of their loved ones, many of whom are at risk of abuse and torture.

“Some are currently held in solitary confinement and incommunicado detention.”

The letter said the King’s intervention “could profoundly impact the lives of individuals who are imprisoned solely for exercising their rights and potentially save those facing imminent execution”.

The letter mentioned human rights activist and scholar Abduljalil Al-Singace and Hassan Mushaima, a Bahraini opposition leader who is now 76, and said both had been detained since 2011.

Earlier this year, King Hamad pardoned 457 prisoners to mark his 25th anniversary as ruler.