Earlier this month the Ladbrokes owner said its cash offer of 2.8bn Swedish krona (£250mln) had been recommended by the Stockholm-listed company.
Alta Fox Capital, based Fort Worth, Texas, said in a statement that it believes the offer of 40 Swedish krona per Enlabs share "is considerably too low and does not compensate shareholders for the excellent growth prospects of Enlabs as a standalone entity".
Alta Fox said it owns 3.34% of Enlabs' total shares outstanding.
It appears that, including Alta Fox, shareholders accounting for more than 10% of Enlabs shares support this view, said broker Peel Hunt.
"If Entain were not able to complete the £250m acquisition of Enlabs it would not throw the group off its overall strategic track, but it would be a headache for the new CEO to inherit once appointed."
Peel Hunt's analysts also pointed out that an article in the Sunday Times, which describes how 30 people reportedly lost a total of £12mln through gambling, "makes a strong case for regulatory change and is likely to attract the attention of policymakers".