Boris Johnson has grudgingly paid up after mounting pressure of an outstanding capital gains tax bill owed to the US government.

Boris Johnson, London's flamboyant mayor, has grudgingly paid up after mounting pressure of an outstanding capital gains tax bill owed to the US government, media reports said.

Johnson, who was born in New York and holds both British and American passports, is among the first high-profile US expats to have been successfully pursued by the Inland Revenue Service, which obliges all US citizens, regardless of where they live, to file a yearly tax return.

The issue highlights how, particularly with the arrival of new tough anti-tax evasion powers under the US FATCA Act, people living outside the US who may have US connections can be caught in the IRS net, even if they hadn't realised their predicament for years.

Asked about the matter by WealthBriefing, Johnson's office declined to comment.

The London mayor made no secret of his uneasy relationship with US tax authorities when he visited his birthland last year to promote his biography on Winston Churchill.

When asked whether he would pay the capital gains tax, levied on the sale of a primary residence, the mayor retorted: “No is the answer. I think it’s absolutely outrageous. Why should I? I haven’t lived in the United States since I was five-years-old. I pay my taxes in full in the United Kingdom, where I live and work.”

Johnson has remained tight-lipped on the exact amount owed from the sale of his first family home in London. According to Land Registry records, he bought the Islington house for £470,000 ($764,690) 1999 and sold it for £1.2 million ($1.85 million) in 2009.

US expats under the same dual citizenship umbrella are often advised to make their spouses owner of the property in question. But this seems a step Johnson had not considered.

The payout will no doubt allow Johnson a more relaxed visit to Boston, New York and Washington next month.

"Even the mayor of London has acknowledged that US citizens have to pay tax that is due as the price of the passport. The IRS expects tax on house sale gains over the exclusion. The US government just wants to help taxpayers settle up with Uncle Sam. Ultimately a cheque (however spelt) beats oratory and bravado," Richard Cassell, a partner in the US-UK tax team of law firm Withers, who advises individuals with interests and tax liabilities in both countries, said in a statement.