The 32-year-old, whose boyband career gave him a share in the group's estimated £31m fortune, said he was devastated by the move, which happened just days before Westlife play their farewell show in Dublin.
Filan was forced into bankruptcy after the collapse of the property business he owned with his brother Finnbarr. Between them they owed more than €5.5m (£4.45m) to Ulster Bank and Bank of Ireland.
The Westlife singer has followed dozens of other Irish debtors who have sought to use Britain's more liberal bankruptcy laws to wipe out their debts. One financial expert managing the debts of Irish people caught up in the country's property crash told the Guardian he will have helped wipe out €1bn of Irish debt in bankruptcy courts by the end of this summer.
Property prices have slumped dramatically across Ireland – in December 2011 they were 50% lower in Dublin than in the boom year of 2006. The latest figures from the Irish central statistics office revealed that 10% of Irish mortgage holders were unable to repay their loans.
It emerged on Thursday that Filan was declared bankrupt this week at Kingston county court in Surrey, a month after his Irish-based property development company went into receivership.
Filan said in a statement he was forced to make the move after exhausting all other options. "Together with a team of financial and legal experts I have spent months exploring all possible alternatives to bankruptcy but to no avail.
"I have worked long and hard to try to reduce my debts, and I am devastated that it came to this conclusion. I now intend to focus on the remaining dates of the Westlife tour and my commitments to the band before looking to rebuild a future for my wife, my three children and myself."
Westlife, currently on their farewell tour, have had 14 No 1 singles in Britain, including Flying Without Wings and Fool Again, and more than two-dozen top 10 hits. In 2010, the group were estimated to have amassed a £31m fortune after selling more than 44m records.
The group will wind up their 14-year career with a show at Croke Park in Dublin on 23 June. Other celebrities have lost millions after investing in Irish property, included actor Colin Farrell, who lost £3m over the past year, according to the Sunday Times rich list.
Irish people with large debts, such as Filan, are turning to British courts to declare bankruptcy because individuals face only one year in financial purdah. In Ireland the penalties last for 12 years.
The ruling in London means Filan's assets are transferred to an official appointed by the court and used to settle his debts, while he is allowed an income to support himself and any dependants. His name has also been published on Britain's insolvency register which gives details of all individual bankruptcy rulings.
The register describes Filan as a singer and gives his address as Cobham, Surrey. He, with his wife Gillian Walsh and their three children, moved to Britain some time ago.
Filan will be discharged on 11 June 2013. At that point, all debts due to his creditors, including his Irish creditors, will be considered cleared. He will not be able to borrow money or act as a company director until he is discharged.
Just weeks ago, Filan agreed a solo record deal with entertainment company Universal. Their farewell concerts at Dublin's Croke Park stadium on Friday and Saturday are already sold out while the cost of hotel accommodation in the city has soared for next weekend as thousands flock to the Irish capital.
