Plans for the UK Home Office’s first mass deportation flight to Zimbabwe have been criticised as “a grubby operation” that risks “delivering democracy activists to political persecution”.

For decades, the Zimabwe government has not accepted people being forcibly returned from the UK, meaning Zimbabweans who sought asylum in Britain were left for decades, starting families and having children.

Its position has shifted in recent years and British and Zimbabwean officials met on 23 June to agree a deal on returns.

Many of the Zimbabweans in immigration detention with tickets for the charter flight, planned for Wednesday, say they fled their home country as a result of having campaigned for human rights in the southern African country and against its former leader Robert Mugabe.

The Guardian understands the Home Office is hoping to deport 50 Zimbabweans on Wednesday but far fewer are likely to be onboard as a result of a number of high court challenges against the removals. The supply of escorts to remove people has also been disrupted after dozens of workers from the contractor Mitie were understood to have been told to isolate amid soaring Covid cases.

The Zimbabwe flight is the first in a series of long-haul mass deportations the Home Office hopes to run in the coming weeks, with flights to Jamaica, Vietnam, Nigeria and Ghana also planned.

The Home Office said it had deported about 700 foreign national offenders so far this year, a substantial drop on previous years. With some travel restrictions easing, it is hoping to increase the numbers removed.

Several Zimbabwean detainees at Colnbrook immigration removal centre near Heathrow said they were terrified of being forcibly returned to their home country. It is understood that people with criminal convictions and those who have overstayed visas have been chosen for the flight.

One man, who has convictions for fraud and driving offences and has been in the UK since 2005, said: “All the Zimbabweans who fled to the UK opposed the government. The UK government does not care what will happen to me if I’m sent back home. I was very politically active against the government there. I’m well known to the authorities because of this and I will really be at risk on return.”

Bella Sankey, the director of the charity Detention Action, condemned the planned charter flight. She said: “Priti Patel’s programme of secret mass expulsions and this particular flight risks delivering democracy activists to political persecution. Detention Action is supporting several people due to be deported in this grubby operation. All have been here for decades and many have several children.”

Annie Viswanathan, the director of Bail for Immigration Detainees, said: “We’re really concerned that the government is determined to go ahead with this flight despite the situation in Zimbabwe. One client who came to the UK feeling political persecution told us they fear for their life if they are deported.”

A spokesperson for Mitie said suitable precautions against the spread of Covid were being taken. They said: “The safety and wellbeing of our employees and detained individuals is always our top priority and we have robust measures in place to ensure a safe environment for all. This includes detailed risk assessments and procedures which have been developed in line with the latest government guidance, as well as the use of PPE such as face masks.”

A Home Office spokesperson said: “We make no apology for seeking to protect the public by removing serious, violent and persistent foreign national offenders. We have removed more than 700 criminals this year, with a combined sentence of more than 1,500 years in prison.”