British Airways has threatened to increase the penalties for cabin crew who continue to strike while halving the punishments for those who return to work, according to a staff email seen by the Guardian. The airline said workers who continued to strike would lose bonuses and staff perks for up to two years.

About 2,900 crew members in BA’s mixed fleet who belong to the Unite union are embroiled in an industrial dispute with the airline, which it says offers “poverty pay” to its cabin crew. The airline has faced 11 days of strike action so far this year over the dispute, with a further eight days scheduled over the next fortnight.

In a note circulated to staff, the airline warned of significant increases in sanctions to those who joined the forthcoming strikes. Unite accused the airline of having “bullied and bribed” staff in an attempt to break the strike.

BA previously warned staff in an email that those who participated in strikes in the first fortnight of the month would lose any bonus for 2016 and would see their 2017 bonuses affected. They would also lose three months’ worth of the “My Incentives” payments scheme, as well as perks including staff travel and discounted travel for family and friends for a year.

Unite said the withdrawal of the My Incentives payment and bonus could amount to up to £850 a year. Basic pay in the mixed fleet, which all new BA recruits join, starts at about £12,000, though the airline says the crew earn a minimum of £21,000 after allowances and bonuses. Unite says cabin crew earn £16,000 a year on average.

Following Unite’s announcement on Tuesday of four additional days of strikes, BA emailed members of the mixed fleet offering a “one-off incentive to resume normal working”. They would receive 50% of any bonuses they had lost through previous strike action and would see the loss of perks such as staff travel and discounts for friends and family limited to a period of three months.

But the airline added that anyone who continued to strike would “see a loss of My Incentives payments for the whole of 2017, complete loss of bonus for 2016 and 2017, and loss of staff travel benefits and Hotline [discounted booking for friends and family] access for 24 months.”

The offer is understood to be a highly unusual way of handling the strike. One observer, who asked not to be named, said it risked poisoning industrial relations with the company.

Unite regional officer Matt Smith said: “Punishing workers for standing up against poverty pay in the first instance and now trying to bribe them back to work if they repent is extraordinary behaviour from an airline that once described itself as the world’s favourite. Our members will not be bullied or bribed into not striking by British Airways.”

He added: “Rather than going to such extreme lengths with such extreme behaviour, we would urge British Airways to sit down with us at Acas to reach a negotiated solution.”

A BA spokeswoman said: “Unite has decided to press ahead with strike action. We have said that we will fly 100% of our customers to their destinations and so are taking these steps to maximise support for our operation. We want to encourage our colleagues to come to work and therefore have repeatedly explained the consequences of taking strike action to them.”