If so, these companies will have to seek authorisation from The Pensions Regulator (TPR) or wind up these schemes.
Master trusts have until the end of March 2019 to apply for authorisation with TPR and comply with new rules which mean these schemes will have to hold enough capital to cover 'worst-case scenario' costs.
This would include transferring pension members to another scheme or of winding up, without charging members.
Ian Neale, director at technical pensions legislation specialist Aries Insight, said cluster schemes could potentially still be considered within the scope of these new regulations.
These are defined by the Department for Work and Pensions as schemes where the same company provides insurance, administration and trustee services, there are no other trustees, and the scheme is not otherwise classified as a master trust, he explained.
Mr Neale said even if these legacy schemes hadn't been marketed, promoted or actively offered for years, there was still risk of the plan being brought under the master trust umbrella because of the regulations’ updated definition.
He said: "This is not an isolated issue and many insurers could be caught out by such legacy schemes being treated as a master trust in this way.
"With the regulator potentially taking up to six months to review applications and the authorisation deadline in April, this is an issue which needs to be addressed as a matter of urgency – or the number of those closing their doors as a result of this process could continue to rise quite dramatically."
He added: "With so much industry attention being devoted recently to the master trust authorisation process, which began on 1 October, concerns have arisen around the new expanded definition outlined in the final regulations made a few days earlier.
"While many have been focused on whether the known master trusts are prepared for the new authorisation regime, it seems that this concern should be shared by a larger group."
The government imposed a wider net on the authorisation process to stop the development of schemes which adopted a structure with the same risks as a master trust without being authorised.
In its response to the consultation on the new rules, the DWP acknowledged some responses warned some schemes would be forced to wind up if they were caught in the regime but it kepts its original requirements.
It said: "The new requirement for authorisation will require the schemes to demonstrate that they comply with the five authorisation criteria and the government recognises that this is not an insubstantial hurdle to overcome."
Kate Smith, head of pensions at Aegon, confirmed cluster schemes were included in the new rules, but said TPR made the industry aware of this earlier this year.
She said: "It appears that some older arrangements may inadvertently be caught by this wider definition even though the intention was never to be a master trust.
"Pensions providers need to urgently review whether any of their legacy schemes are caught by the cluster scheme definition and open discussions with TPR.
"If it is found that that legacy schemes are caught by the regulations its highly likely they will be wound up rather than apply for authorisation."
maria.espadinha@ft.com
