Resident doctors have accused Keir Starmer of damaging the prospects of a deal to end their pay and jobs dispute by threatening to cut 1,000 new jobs for medics in the NHS.

The claim from the British Medical Association leaders came just before the Thursday deadline given by the prime minister for the union to accept the government’s final offer.

Barring a late change of heart by the BMA, resident doctors in England will stage a six-day strike from 7am on Tuesday. The union’s resident doctors committee last week rejected the government’s detailed plan to end their long-running dispute.

It included a pledge to increase the number of places available in specialist medical training by up to 4,500 over the next three years to help more early-career doctors start training in their chosen speciality. About 1,000 doctors were due to step into those roles from this August.

However, Wes Streeting, the health secretary, has told the BMA that those 1,000 extra slots in specialist training will be scrapped unless it accepts the deal.

Talks will continue on Thursday aimed at averting next week’s strike. But there is little optimism on either side that it will be called off. NHS bosses fear it will severely disrupt services, especially as it will fall during the Easter school holidays when many staff will be off.

Dr Jack Fletcher, the chair of the BMA’s resident doctors committee, wrote in a letter to the health secretary on Wednesday: “The political rhetoric – threatening to remove training places – coupled with the way the government has communicated the offer, has needlessly and avoidably inflamed the dispute, ultimately pushing the chance of a deal further away. A final offer followed by threats that parts of the offer may be withdrawn is not the way to end this dispute.”

The threat to remove 1,000 of the 4,500 jobs has heightened fears among residents that they will not be able to progress into specialist training this year, he added.

Starmer wrote in the Times on Tuesday that the BMA would be reckless if it did not accept the government’s “historic deal”. Rejection would also mean resident doctors miss out on an above-inflation pay rise this year.

Starmer gave the union until Thursday to rethink its position and accept the offer. Streeting also wrote to all of England’s 75,000 resident doctors advising them to accept.

The Department of Health and Social Care (DHSC) did not immediately respond to Fletcher’s letter, which outlines the issues on which the BMA and government remain far apart. They include whether resident doctors should receive £700m extra in “progression pay” over one, two or three years – and the BMA’s demand for pay deals until 2029 to insulate them from the effects of inflation.

The BMA opened a new front against the government this week by deciding to ballot other types of hospital doctor, including consultants, about striking over “inadequate”pay and threatening to coordinate strikes.

A DHSC spokesperson said: “It is disappointing that the BMA has decided to press ahead with strikes next week, despite conversations we have been having in recent days in a bid to protect the NHS from strikes.

“This government offered resident doctors a generous deal to improve their pay, career progression and working lives that would have seen resident doctors on average 35.2% better off than they were four years ago.

“Because the BMA resident doctors committee has not agreed to call off these strikes and put an offer to members, we will now not be able to deliver the 1,000 extra training places which the BMA asked for.”