A violent racist with a history of mental health problems who admitted a terrorist offence and stockpiling equipment to bomb a mosque has been jailed for four years, one of the lowest sentences for someone plotting a potentially deadly attack.

Steven Bishop, 41, amassed commercial grade fireworks and other equipment as he formed the intention to build a homemade firebomb he would remotely detonate to attack the Morden mosque, in south London. He may serve just over another year in prison before his release on licence.

He claimed his plot was “revenge” for the Manchester arena bombing in 2017 in which 22 people died, but was stopped in October 2018 after a counsellor he saw for addictions reported Bishop to police after he told her of his plans.

Steven Bishop. Photograph: Metropolitan police/PA

Bishop had pleaded guilty earlier this week at Kingston crown court to two offences, one under the Explosive Substances Act 1883 and the other, having material useful for terrorism.

Concerns have been raised about the length of sentence, as British authorities try to show they take extremist rightwing violence as seriously as Islamist attacks.

Lord Carlile, the former independent reviewer of terrorism legislation, told the Guardian: “It is one of the lowest sentences for this kind of offence.

“I would expect the prosecution would consider appealing this sentence on the grounds that it is unduly lenient. I would be surprised if that was not the view of the Crown Prosecution Service.”

A spokesperson for the Muslim Council of Britain said: “There will be deep concern amongst many that this man was given a relatively light sentence even though he was poised to murder people.

“We learned from the case that Mr Bishop was driven by a hatred of Muslims. We hope we will be able to learn how the punishment fits this particularly serious crime.”

The Guardian understands the length of sentence also caused surprise among counter-terrorism investigators.

When detectives from Scotland Yard’s counter-terrorism command searched Bishop’s room in October 2018 they discovered fireworks as well as fuses, a remote control and igniter. He had also ordered a firing device.

He had placed virtual private network (VPN) apps on his phone to hide his online activity, which included researching explosive detonators and the target which was the Morden mosque.

Bishop, who had significant mental health problems, had stashed parts for making improvised incendiary devices at his mother’s home, hidden in a suitcase in the garden shed.

Bishop made notes detailing recipes and methods to make explosive substances.

The sentence was passed by Judge Lodder QC, who is experienced in similar cases.

In 2015 the same judge sentenced Trevor Mulindwa, an Islamist terrorist with mental health problems to six years for wanting to flee abroad. Mulwindwa, who had been treated in a mental health unit, had talked of being a suicide bomber, but had not bought bomb parts.

Sentencing Bishop, Lodder said: “The detonation of one or more of these fireworks at Morden mosque may have risked the lives of those nearby.”

“It is submitted that you are so far out of your depth that it is absurd and that the mosque was never truly in danger. In that context you are contrasted with a determined and dedicated terrorist. But terrorist acts are not limited to those who do not have similar vulnerabilities to you.”

He added: “The seriousness of the offending is reduced by your medical history, but I do not find that your condition at the time of offending was substantially reduced by mental disorder or learning disability.”

Prosecutor Dan Pawson-Pounds told the court: “Much as he never formed the final intent actually to carry out the act against the mosque at any particular point, he accepts that he carried out acts preparatory to it, and in contemplation of it.”

Bishop has had lifelong psychiatric disorders including paranoid schizophrenia, ADHD, autism spectrum disorder, a possible learning difficulty and also has experience of drug abuse. His 18 previous convictions include racially aggravated common assault.

Bishop’s barrister, Timothy Forte, denied his client’s action were fuelled by extremist rightwing beliefs. His web browsing history showed him viewing a pro-Brexit site and one instance of far-right material. Forte said he was moved by a victim of the Manchester attack, Saffie Roussos, aged 8: “It is only ever about Saffie. There is no expression of anti-Islamic feeling.”

“He’s not seeking to obtain a white Britain or a Muslim-free country. He was seeking in his non-functioning manner to exact revenge for the death of an eight-year-old girl and the crown can’t demonstrate an ideological cause.”

Bishop received benefits of around £1,200 a week and used the money to buy and consume drugs.

Bishop pleaded guilty to possession of an explosive substance with intent, in breach of the Explosives Act. The charge he admitted said that Bishop had by “29 October 2018, unlawfully and maliciously made or had in his possession or under his control an explosive substance with intent by means thereof to endanger life, or cause serious injury to property”. He received four years imprisonment for that charge, and an additional one year on licence because the offence was connected to terrorism.

Bishop also admitted an offence under section 58 of the Terrorism Act 2000. He accepted he had “made a record of information of a kind likely to be of use to a person committing or preparing an act of terrorism”. He received two years for that count, to run at the same time as the explosives offence.