The consumer champion found many policies exclude "plausible and often expensive scenarios", such as new lockdowns in the UK or destination countries.
It looked at 263 providers and gave them a rating of "basic", "low", "superior" and "complete". HSBC Select and Cover and Barclays Travel Pack were the only two policies rated as "complete".
They each protect customers against cancelations due to changes to Foreign Office advice and government lockdowns. The plans also offer protection for travellers who test positive for Covid, or are told to self-isolate.
A further 85 policies, including those from AA, AXA and Staysure, were ranked "superior", providing cancellation cover for individuals having to self-isolate without a positive test, but not for changes in FCDO advice.
More than half of the policies (142) were ranked "low", including options from Nationwide, Admiral and the Post Office.
Which? said these only offered some cancellation cover, which does not go as far as covering those who are forced to cancel in the event of needing to self-isolate.
There were 34 policies ranked "basic", which do not provide cover for cancelling a trip if a traveller contracts Covid. These providers include Direct Travel, Esure and Sheila’s Wheels.
Gareth Shaw, head of Which? Money, said: "As the removal of Portugal from the green list shows, last-minute disruption to holiday plans can happen – and our research shows that many travel insures don’t offer much protection if it does.
"The government should work with regulators to ensure that travellers, should they choose to go abroad, are given clear information about what they will and won’t be covered for – and make sure that providers don’t make bold and confusing claims about their cover without being clear about the limitations."
