Shadow Chancellor John McDonnell has joined Islamist groups calling for Prime Minister David Cameron to cancel a visit to Downing Street by Egyptian president Abdel Fatah al-Sisi. He is one of 55 signatories of the letter that alleges Sisi is a military dictator responsible for a "regime of terror".

Labour's shadow international development secretary Diane Abbott and the Green Party's MP Caroline Lucas have joined UK-based Muslims with alleged links to the Muslim Brotherhood, the Islamist organisation ousted from power in a coup led by Sisi in 2013, in signing the letter.

The invitation was confirmed by Downing Street the day after the death sentence for Mohammed Morsi, the Muslim Brotherhood president toppled by Sisi, was upheld in an Egyptian court.

In the letter sent to The Guardian, the authors write: "We believe it violates the British values, which the government claims to champion, to welcome a ruler who has overthrown an elected government and instituted a regime of terror that has thrown back the cause of democracy in Egypt and the wider Middle East many years.

"While not necessarily supporting deposed president Morsi or the policies of his Freedom and Justice party, we note that he was democratically elected, and that his removal from office was effected by means of a military coup led by Sisi. Since then Sisi's military-directed regime has massacred thousands of civilians."

The authors claim the Sisi regime has taken part in widespread human rights violations, including execution of hundreds of Muslim Brotherhood activists, the arrest of journalists and the suppression of opposition political activity.

Cameron's government has chosen to engage with Sisi's government, and in 2014 ordered a review of the UK activities of the Muslim Brotherhood in the UK, led by Britain's ambassador to Saudi Arabia Sir John Jenkins. Cameron has also pledged to broaden the UK's anti-terror strategy by promoting "British values" and combating Islamist ideas alleged to foster terrorism.

The signatories include filmmaker Ken Loach, Daily Mail columnist Peter Oborne, and Unite Union chief of staff Andrew Murray as well as figures from Islamist groups.

Anas Altikriti, CEO of the Cordoba Foundation, which has been linked to the Muslim Brotherhood, and the Egyptian Revolutionary Council, a coalition of Egyptian opposition figures opposed to the Sisi regime, including figures from the Muslim Brotherhood, are among those who signed.

Altikriti has expressed support for the Muslim Brotherhood as well as militant group Hamas, which is listed as a terror group by the UK and US. In 2008 Cameron described the Cordoba Foundation as a "front for the Muslim Brotherhood", a charge denied by the organisation.

In July, a series of figures associated with the Cordoba Foundation, including Altikriti, had their bank accounts closed by HSBC, which said the clients fell outside the bank's "risk appetite".

Speaking to IBTimes UK, Altikriti said that one of the chief drivers of extremism is the UK government's support for rulers like Sisi, not Islamist organisations. "Rolling out the red carpet for someone as bad as Sisi is gruesome, it is unbecoming," he said. "These days we are all in this war together fighting against extremism and terrorism what really fosters terrorism and extremism and recruits for the cause of extremists is the fact that we do away with democracy whenever it suits us".

Daud Abdullah, of the British Muslim Initiative, is another signatory. In 2009, Abdullah signed the controversial Istanbul Declaration, calling for attacks on foreign navies if they halt arms transportations to Hamas.

The Muslim Association of Britain, which was previously headed by Altikriti, has signed the letter. The organisation has declared that it "shares some of the principles of the Muslim Brotherhood". Dawatul Islam and the Islamic Forum of Europe, whose founding members belonged to the Bangladeshi Islamist group Jamaat-e-Islami, are also signatories.

A Downing Street spokeswoman would not confirm that Sisi is due to arrive in the UK next week, but defended Cameron's invitation.

"The prime minister has invited President Sisi to Downing Street to discuss how to work together on areas of mutual interest, including combating terrorism in Egypt and the region, and bringing stability to Libya," she said.

"The stronger our working relationship, the more able we are to have necessary and frank discussions about issues on which we disagree." It said the government will publish the findings of the Muslim Brotherhood review before the end of the year.