Andy Burnham activated submarine mode today as Keir Starmer and Kemi Badenoch rowed about what he should do about defence.The PM-in-waiting was nowhere to be seen at the weekly PMQs session in the Commons, despite being the main topic of debate.Less than three weeks before he could take over in No 10, the Makerfield MP has not been spotted in public since giving a speech in Manchester on Monday.He exited that without taking questions from the media, leaving the country little clearer about details of his policy platform.Mr Burnham did post a video of himself last night judging AI-generated posters depicting his proposed 'No 10 North'. Commons records show he voted last Wednesday, but has otherwise kept a low profile since being sworn in on Monday last week and taking a 'mass selfie' with fawning Labour MPs. At a bad-tempered PMQs, Sir Keir dodged questions on whether Mr Burnham had agreed to fill a near-£5billion 'unexploded bomb' in his new defence plans.The outgoing premier merely insisted that 'any Labour prime minister' would stand behind his package.Mrs Badenoch said the Government was scrabbling for money because 'he was too weak to cut welfare when he had the chance'. She argued that the only ways of freeing up funding were tax rises, cuts to services, or trimming welfare. Critics have warned that the £15billion funding boost over four years in the Defence Investment Plan (DIP) is 'too little too late'.But it has emerged that the source of £4.7billion of the cash will only be decided at the next Budget, by which time Mr Burnham will almost certainly be PM.Billions of pounds more will come from cuts to other departments, many of which have not been defined. Downing Street admitted that could affect hospital and road projects.The IFS think-tank warned that tax rises or curbs to services are inevitable to meet the costs.Mr Burnham was briefed on the DIP but does not seem to have learned of the funding gap until yesterday, when a Treasury document was released. One Labour source who has been working with the Burnham team said they were 'furious'. At a bad-tempered PMQs, Keir Starmer insisted that 'any Labour prime minister' would stand behind his package The PM repeatedly ducked demands from Kemi Badenoch to say whether his successor had signed off Burnham was briefed on the Defence Investment Plan but does not seem to have learned of the funding gap until yesterday, when a Treasury document was released