A father of two has been convicted of a terror offence for arranging to travel to Syria to join a jihadist training camp.

Mashudur Choudhury, 31, is one of at least five men from Portsmouth who travelled to the conflict last October with the alleged intention of joining al-Qaida-inspired rebel groups fighting Bashar al-Assad's forces. He is the first British man to be convicted of a Syria-related terror offence since the conflict began.

His conviction will be seen as a victory for the police's counter-terrorism strategy of trying to persuade relatives, friends and associates to come forward if they suspect that an individual is planning to join the fighting in Syria.

Choudhury, dressed in a green polo shirt, did not react as the jury returned a unanimous verdict after four and a half hours of deliberation. The judge, Mr Justice Dodgson, warned him he faced a substantial custodial sentence, and adjourned the case for probation reports. Choudhury will be sentenced on 13 June.

Police sources said they were tipped off from within the Muslim community in Portsmouth that Choudhury and four other men had left for Syria last October. The men allegedly decided to go to training camps after another associate, Iftekhar Jaman, 29, had travelled to fight with the Islamic State of Iraq and Syria (Isis) earlier last year.

Jaman, who became a celebrated figure in some circles on social media, encouraged others to follow him and take up arms against the Syrian regime. He was killed in December fighting with Isis, shortly after he had given an interview to BBC Newsnight.

Syrian rebels attend a training session in Maaret Ikhwan, near Idlib, Syria. Photograph: Muhammed Muheisen/AP

The jury at Kingston crown court convicted Choudhury after a trial in which they were presented with his exchanges on social media, Skype and text messages, which the crown said proved he intended to go and join a terror training camp in Syria.

Assistant chief constable Brendan O'Dowda, who heads the south-east counter-terrorism unit, welcomed the verdict. "Today's conviction sends out strong message to anyone considering engaging in terrorist activity, be that in the UK or overseas, that we will pursue you and your networks," he said. "The ongoing conflict in Syria is terrible for us all to see. Great sympathy is felt for those victims of that conflict. If anyone has a genuine desire to help then they are encouraged to offer aid and support by donating through charities supporting the relief operation.

"Syria is a very dangerous place, best described as a cocktail of high risk and threat. Anyone thinking of travelling to fight jihad against the Assad regime, think again. You are likely to be killed or kidnapped and if you return to the UK you are highly likely to be arrested. We hope that this case offers reassurance that we will work tirelessly to identify and tackle the threat posed from terrorism. The safety and security of all our communities across the UK is paramount."

The prosecutor Alison Morgan told the jury they did not have to make a decision about the rights and wrongs of what was going on in Syria in order to convict Choudhury. "The prosecution's case against this defendant is that he was travelling to Syria to train to fight in pursuit of an ideological or religious cause. You are not being asked to determine who is right and who is at fault in Syria."

The jury was shown CCTV footage of Choudhury and the four other men from Portsmouth at Gatwick airport en route to Turkey. Of the five, only Choudhury returned. The other men are believed to be fighting with jihadist groups against the Assad regime.

The crown presented Skype exchanges with Jaman in which Choudhury spelled out his plan to join him in Syria and fight. On one occasion he asked about what kind of regime he would undergo while training.

"I read something on a website stating four months on average. Sounded proper hardcore, like running for 10km without stopping," Choudhury said.

In one exchange he suggested a name for the group from Portsmouth in Syria – the al-Britani Brigade Bangladeshi Bad Boys – while in others he asked how he and the other men would get across the border to Syria and whether they would all be trained together.

Jaman told him in one exchange: "There may be a chance for us to learn sniping together, battlefield training." To which Choudhury replied: "Yeah man, that sounds sick."

He told his wife: "What good is a husband, father, brother that sits in comfort, sleeps in comfort, eats in comfort but neglects the cause of women being raped, children being attacked, mothers being decapitated, and daughters being murdered?"

Choudhury flew out with the other four men from Portsmouth on 8 October last year. The crown admits it will never know exactly what he did while he was in Syria, but it relied on his social media and Skype exchanges to argue that he intended to join a terror training camp.

Choudhury had been under pressure from within his community because he was being blamed for encouraging the group to travel to Syria. Leaders of the Jami mosque in the city feared that as Choudhury was a youth worker, his actions might overly influence vulnerable young people.

He was arrested as he landed at Gatwick and later charged with conduct in preparation of terrorist acts, contrary to section 5 (1) of the Terrorism Act 2006.

Police and the security services estimate that between 400 and 500 men from the UK have travelled to Syria and either returned or remain in the country. Choudhury's trial was the first of a handful of forthcoming prosecutions of British men – and two young women – for Syria-related terror offences.

Portsmouth district commander, Supt Ali Heydari, said: "This is a unique case for Portsmouth and in fact Hampshire and the Isle of Wight as a whole. We have very good relationships with key figures within the city and work with them to share information. We all have a common aim to prevent radicalisation in all forms.

"In this case we received information from the community which was acted on and led to today's result. We would like to thank Portsmouth residents for continuing to report anything they are concerned about to us."

David Williams, Portsmouth city council's chief executive, said Choudhury had been employed following an enhanced criminal records check, and held a junior position for a year as a racial harassment caseworker, during which he was seconded as a part-time community development worker for six months.

"Portsmouth is a diverse and tolerant city, as a council we pride ourselves on building relationships with all community groups," he said. "Community safety is an important service of which preventing extremism is one element, but alongside our partners the main aim is to create one Portsmouth, a city everyone feels part of. We will continue to build engagement and trust with our Muslim community."