There are estimated to be 159,000 US expats living in the UK - that is, people born in the US - as of November last year.
Many have taken up residence in the UK on a semi-permanent basis and have been living here a long time.
But what many may not realise - especially those for whom the UK has been their main residence for much of their lives - is that the US tax authorities can catch up with them and land them with an unexpectedly large tax bill.
The tax regime, run by the Internal Revenue Service (IRS), can cover many aspects of a client's affairs, but is especially applicable to investments and property assets, and if one is planning on giving up one's citizenship, then it could be one's entire belongings.
The most important point to remember is that if an adviser has someone with a US passport or green card on their books, then the earlier they can get some planning done, the better, as the IRS does not look kindly on anyone who appears to be trying to avoid tax.
Sigita Dromantaite, director at US and UK Eagle Taxation, says: "We're talking more than 50 per cent or 40 per cent of tax (including penalties and interest) - both US citizens and green card holders - and these are the areas where they usually stumble upon, and they don't have enough knowledge and get into investments that are not the best for them."
The US tax authorities treat anyone with a US passport or green card as being liable for tax, regardless of where they are living in the world. Sometimes they will be subject to UK tax laws as well, and generally speaking, if the same asset is subject to a tax charge, the bill will revert to whichever regime has the higher rate of tax.
The tax charges in the US are to some extent comparable to the tax charges in the UK; it is when the IRS believes that the individual is trying to avoid paying tax is when the charges get higher.
And some investment wrappers are completely unrecognised by the US tax authorities as tax-free investments, such as Isas, as well as many investment funds. So it is not worthwhile leaving assets in an Isa as they will still be hit with a tax charge.
Ms Dromantaite says: "Isas are quite good in the UK, but they're absolutely rubbish for US citizens, because they have to pay taxes on their gains and interest and dividends, or if a timely election is made, on unrealised gains."
Similarly, many investment funds available to UK investors are unsuitable for American clients, because they are deemed to be 'PFICs', a passive foreign investment company, and will be taxed differently.