Cricket South Africa has admitted that it is powerless to prevent players joining English counties on Kolpak deals, as recent Test cricketers continue to turn their backs on international careers. The loss of high-quality cricketers in the country has fuelled fears of a player drain, similar to that which the Springboks rugby union side has faced.

At least five players who have played Tests since November 2015 have joined or are set to join English counties as they flee quotas, seek better pay and look to move before Brexit could nullify the EU ruling that allows South Africans to play in England as non-overseas players.

The Guardian understands that the off‑spinner Dane Piedt, who played against England this year, and at least one squad member for the recent tour of Australia (although not one who played in the Test series, which the Proteas won 2-1) are seeking to move to English cricket. Kolpak players can play domestic cricket in South Africa in the English winter, but forfeit their international careers while under contract in England.

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Already, the spinner Simon Harmer has joined Essex, Stiaan van Zyl has moved to Sussex and Derbyshire have signed Hardus Viljoen to spearhead their pace attack; all three players are in their 20s. In October, the former under-19 international Colin Ackermann joined Leicestershire, too.

Kolpak players in county cricket have long been a contentious issue in that they dilute opportunities for England‑qualified players even though they raise the quality. In a 2008 game between Northamptonshire and Leicestershire, 12 of the 22 players were on Kolpak deals. As eligibility regulations have tightened, the number of players registered as Kolpaks has reduced in recent years, with the number between 3% and 4% during the 2016 season.

Piedt, who is 26 and played the most recent of his seven Tests in August, believes he is unlikely to be signed by a county as an overseas player and is therefore happy to play as a Kolpak. The paucity of high-quality spin in the County Championship means that, like Harmer, he is well placed to earn a deal, although he has not yet formally spoken to any counties.

The arrangement provides players with greater job security, but also greater remuneration; with the Rand weak, Van Zyl – who played 12 Tests before being dropped in August – has a contract with Sussex that is understood to be worth roughly three times as much as his deal with the Cobras in Cape Town.

“We cannot restrain individuals from plying their trade,” Haroon Lorgat, the chief executive of Cricket South Africa, said. “Over and above our regular reviews of the domestic playing rules and regulations to ensure it is fit for purpose, we will continue to rather focus on making sure that we have the best systems to produce the best cricketers in numbers.

“We have realised a long time back that the world is a global village with people very mobile. Like in every other country and across every other profession, South African citizens will venture abroad to take advantage of stronger currencies and employment opportunities.

“This mobility affects all professions and is not limited to cricketers who ply their trade in English counties.”

The motivation to leave at this time is heightened by uncertainty over the perceived impact of Brexit. Players who have already signed Kolpak deals – Van Zyl and Viljoen are both contracted until 2019 – believe they will be unaffected, but that things could change in 2017.

“I have been told that players are choosing Kolpak deals now for fear of losing this opportunity once Brexit is implemented,” Lorgat said, but the ECB told the Guardian it was too early to know the impact of the referendum vote.

“Regarding the wider impact of Brexit,” said an ECB spokesperson, “it is too early to predict the outcome in this area. We await more information on the legal situation post-Brexit and will then need to hold further detailed discussions with our stakeholders, the government and other sporting bodies before we can comment further on the potential implications for cricket.”

Lorgat was less forgiving of those citing paths blocked by quotas and transformation targets for their departure. CSA this year introduced a system requiring national teams to contain an average (across a year) of six players of colour, two of whom would be black African. Piedt would be the first non-white player to sign a Kolpak deal since the new targets were brought in.

“Any player citing transformation or targets is looking for a soft excuse as he or she might not be prepared to work hard enough to fight for a place in our representative teams,” Lorgat said.

“We only select the best as was evident in our recent successes against Australia. You don’t whitewash Australia or beat them seven times in a row without choosing your best.”