Early in 2016, MPs at Westminster are due to take a momentous decision: whether or not to renew the UK’s Trident nuclear weapons system. The trouble is, judging from what we’ve learnt in 2015, the decision has already effectively been made – and it will bring a host of problems in its wake.

The House of Commons is expecting to vote in the next few months on what the Ministry of Defence (MoD) calls “Main Gate”. This is the major decision on whether or not to authorise multi-billion expenditure on replacing the four ageing Vanguard submarines that carry Trident nuclear missiles in and out of the Faslane naval base on the Clyde.

But it has become increasingly clear that, following an earlier “Initial Gate” decision in 2011, major sums of money have already been committed. In February 2015, the Sunday Herald reported that £4.2 billion was going to be spent on designing new submarines, reactors and missile compartments before next year’s parliamentary decision.

The figure came from the UK government’s National Audit Office. The MoD was roundly attacked by the Scottish National Party and the Greens for pre-empting the decision to renew Trident.

“It is utterly unacceptable that over £4 billion will be blown on replacing Trident nuclear weapons before parliament actually decides on whether or not to even give it the go ahead,” said the SNP’s Westminster leader, Angus Robertson, at the time.

The Green MSP, Patrick Harvie, said it was “absurd” to spend so much money in advance of taking a major investment decision. The MoD, however, insisted it had “been transparent with the costs whilst protecting our commercial position.”

Then in April 2015, the Sunday Herald reported that the MoD had launched a series of studies into how the Clyde naval bases at Faslane and Coulport could be upgraded to accommodate “successor” submarines. Again the moves were condemned as undemocratically pre-empting next year’s Main Gate decision.

At the same time as forging ahead with renewing Trident, the MoD was also running into serious problems maintaining the existing system. Its internal watchdog, the Defence Nuclear Safety Regulator (DNSR), reported that the ministry was struggling to recruit enough skilled engineers to keep its bombs and submarine reactors safe.

The shortage of suitably qualified nuclear staff was “the principal threat to the maintenance of safety in the defence nuclear programme”, a DNSR report disclosed in March 2015. The staff shortages were caused by competition from the nuclear power industry, which the UK government was trying to greatly expand.

MoD latest figures suggested that one in ten nuclear safety posts were vacant, and the vacancies were increasing. The MoD was missing 165 suitably qualified and experienced nuclear personnel on 31 March 2014, compared to shortfalls of 133 in March 2013.

There has also been a sharp increase in radiation safety incidents at Faslane and Coulport. In March 2015 the MoD disclosed in a parliamentary answer that the number of “nuclear safety events” at the two bases leapt by more than 50 per cent from 68 in 2012-13 to 105 in 2013-14.

The total for 2013-14 is by far the highest for at least the last six years. Most of the incidents – 99 – involved the reactors that power Trident and other Royal Navy submarines, with a further six involving nuclear weapons.

The issue of nuclear safety only really sunk into public consciousness, though, after the Sunday Herald revealed in May this year that a Royal Naval submariner, William McNeilly, had gone on the run after claiming that Trident was a “disaster waiting to happen”.

The 25-year-old whistleblower from Belfast had been on patrol with the Trident submarine, HMS Victorious. He posted online an 18-page report alleging 30 security and safety flaws on Trident submarines.

Trident missiles were vulnerable to a terrorist attack that “would kill our people and destroy our land”, he claimed. Infiltrators had “the perfect opportunity to send nuclear warheads crashing down on the UK.”

Most serious of all, McNeilly highlighted evidence that an inherent flaw in Trident missiles could lead to fires, explosions and widespread radioactive contamination. He photographed a top-secret naval safety manual, which warned that the “chief potential hazard” from a live missile was the “accidental ignition” of solid rocket fuel.

This could cause the warheads’ conventional high explosives to detonate and scatter plutonium and other toxic materials “over a wide area”, it said. This had been flagged up as a design flaw by US experts in the past, but had not previously been acknowledged by the MoD.

The day after his accusations were first carried by the Sunday Herald, McNeilly handed himself in to the police at Edinburgh airport. He was then passed to the naval authorities, which eventually decided to dishonourably discharge him from the service.

He attacked “military deceivers” and naval “spin doctors” for downplaying his allegations. “It is shocking that some people in a military force can be more concerned about public image than public safety,” he said.

The MoD’s initial response to McNeilly’s allegations was to dismiss them as “subjective and unsubstantiated”. After a rapid inquiry it concluded that many of his allegations were “factually incorrect or the result of mis- or partial understanding".

In defending the MoD’s response before parliament, however, defence ministers had a startling new political reality to confront. At the UK general election on May 7 2015 Scotland returned 56 SNP MPs and one Labour MP opposed to Trident.

The SNP chose to make one of its first debates in the new parliament about McNeilly’s allegations. The party’s new MPs were scathing about the MoD’s failure to address his concerns.

Among them was Brendan O’Hara, who won the Faslane constituency of Argyll and Bute with a majority of nearly 8,500 and ended up being appointed as the SNP’s defence spokesman. “I highlighted what I regarded as the MoD's cavalier dismissal of the serious safety concerns raised by Mr McNeilly pointing to the MoD's long history of secrecy and complacency at the nuclear base,” he said.

“It is bad enough that Scotland is forced to house these weapons of mass destruction. But when such allegations are dismissed in a statement containing just 420 words, can we have faith that that the MoD are taking safety concerns seriously enough and are not indulging in their usual “just move along, there’s nothing to see here” approach to public scrutiny?”

The other political earthquake to shake Trident was the election of Jeremy Corbyn as Labour leader in September by a landslide 60 per cent vote of party members and supporters. A long-term anti-nuclear activist, he is resolutely opposed to renewing Trident.

But he almost immediately came under fire from fellow Labour MPs for saying that, if he became Prime Minister, he would not press the button to launch a nuclear attack. UK Labour Party policy on Trident has since been effectively kicked into the long grass via a policy review.

Labour Party policy in Scotland, meanwhile, has hardened against Trident. Its Scottish conference voted by 70 per cent in November to scrap Trident, describing it as “a mortal threat to humanity’s survival”.

That was quickly followed by a 96 to 17 vote in the Scottish Parliament in which all but one of Labour’s MSPs voted with the SNP to oppose the renewal of Trident. The exception was Jackie Baillie, Labour MSP for Dumbarton, which includes Faslane.

The political sea changes were welcomed by veteran peace activists. “The SNP and Labour were able to work together and enable the Scottish Parliament to take its firmest stance yet, with 96 MSPs voting against Trident,” said John Ainslie, coordinator of the Scottish Campaign for Nuclear Disarmament.

“The shambolic state of the Trident programme has been laid bare. After the revelations of the whistleblower William McNeilly, we have then seen the MoD acknowledge that the renewal programme is late and over budget.”

In October it became clear that the capital cost of Trident had soared from £25 billion to £41 billion. Crispin Blunt, the Conservative MP who chairs the House of Commons foreign affairs committee, put the overall lifetime cost of renewing the nuclear weapons system at £167 billion.

Despite all this, at the moment it still looks likely that the majority of MPs at Westminster, supported by some Labour MPs, will vote to approve Trident renewal in 2016. That will put a severe strain on the union, and on democracy itself.

“A vote on Trident will be a test of parliament’s respect for the democratic wishes of the people of Scotland,” said Ainslie. “Even if, as expected, all but one of Scotland’s 59 MPs vote against Trident, we could see parliament giving a green light to the plan to base these weapons of mass destruction on the Clyde for the next half century.”

In 2015 Trident prompted multiple protests and arrests at Faslane and elsewhere, as it has done for decades. If Westminster does end up giving it the go-ahead in 2016, the demonstrations are likely to escalate.