The company said all trips would be cancelled up to and including June 11, and warned customers wanting cash refunds that its call centres are "incredibly busy".
It had previously suspended its operations up to May 14.
Tui also cancelled its Marella Cruises sailings up to the end of June, and postponed the launch of its river cruises until late November.
A total of nearly 900,000 people have had their holidays cancelled, a spokeswoman said.
The firm told affected customers that they will receive a refund credit for the full value of their holiday.
This can be used to book another trip taking place before the end of October 2021.
Package holiday customers will receive an additional credit worth up to 20% of their booking.
Tui said customers who are "unable to accept" a refund credit can apply for a cash refund, but warned that its call centre staff are "incredibly busy" so waiting times are "considerably longer than usual".
Simon Cooper, founder and chief executive of rival travel agency On The Beach, told the PA news agency last week that failing to pay cash refunds is "a bad idea for everyone concerned" as struggling companies will be forced to "massively increase" their prices for next year's holidays to avoid bankruptcy.
UK laws state that full refunds should be given within 14 days for cancelled package holidays.
Consumer group Which? found that none of the UK's 10 biggest holiday companies or 10 most popular airlines are offering full refunds within the legal timeframe, and some are refusing to provide refunds altogether.
Many travel firms are suffering huge financial losses due to the collapse in demand caused by the coronavirus pandemic.
The Foreign and Commonwealth Office has advised against all but essential international travel since March 17.