Labour’s biggest union backer is giving away a thousand tickets to an event billed as Jeremy Corbyn’s very own Glastonbury, amid serious fears that it will lead to major losses for the party.

Unite announced to members that it was giving away the tickets and even offered free travel to the event in London for those who need it.

It follows recent reports that ticket sales for the Labour Live Festival have been sluggish, with only 2,500 of the 15,000 seats sold for the event at White Hart Lane in north London on Saturday 16 June. Full-priced tickets cost £35.

Some insiders are worried that the party could be in for a seven-figure loss unless action is taken, though Labour officially denies such a loss is on the cards. Unite, whose general secretary Len McCluskey has been one of Corbyn’s most powerful allies, has now stepped in to help boost the numbers.

“If you are quick you have the chance to book a free ticket for the event,” the union said in a message to members. “Unite have 1,000 tickets to give away on a first-come first-served basis … Contact your local Labour party regional office to find out about the free travel services we’re providing.”

The Labour Live event has struggled to attract big-name acts, but it will feature Hookworms, the Magic Numbers, Rae Morris and Jermain Jackman. The event was designed to benefit from the adulation Corbyn received when he appeared at other live events, including Glastonbury last year.

A Labour spokeswoman said: “Labour Live is one of the ways we’re continuing to open up politics to a wider audience and spread Labour’s message about how we can build a society that works for the many, not the few. We welcome the support of the wider labour movement.”