Tui has suspended all holidays for UK customers until the end of June.
The tour operator had previously announced all trips until June 11 are cancelled, but now it has suspended all holidays until the end of the month "due to the ongoing travel restrictions".
Affected customers will be sent a refund credit, Tui says, which can be swapped for cash by submitting a refund request form on Tui's website.
Those who trips planned between July 1 and August 31 can amend their bookings to any date up to October 21, for free, says the tour operator.
Tui follows package rival Jet 2 Holidays in suspending all trips up to June 30.
A 14-day quarantine for international arrivals is due to begin in the UK on June 8.
The Foreign and Commonwealth Office (FCO) has advised against all but essential international travel since March 17.
Major UK tour operators will not run trips until the FCO changes its position.
Health Secretary Matt Hancock appeared to be more optimistic than in the past when asked about the prospect of people being able to go on holiday this summer.
He told BBC Radio 4's Today programme: "I think the answer is let's see where we get to."
Mr Hancock had previously commented that "big, lavish international holidays" were "unlikely" to be possible this summer.
