Whether you need to withhold tax would depend on whether the visiting professional needs to pay tax in Singapore under the provisions of the tax treaty. For example, under the tax treaty between Singapore and United Kingdom, the income derived by the non-resident professional from Singapore will be tax exempt provided:
a. He does not have a fixed base regularly available to him in Singapore for the purpose of performing his activities; or
b. He is not present in Singapore for not more than 183 days in any period of 12 months commencing or ending in the fiscal year concerned.
If he (with no fixed base in Singapore) is in Singapore for a 5-month engagement in year 2008, his income derived during that period will be tax-exempt for Year of Assessment (YA) 2009 (See Chart 1).
: Tax Exempt for YA 2009 as presence in Singapore < 183 days
However, if he comes to Singapore for another 5-month engagement in year 2009, his income derived from the first engagement in 2008 will then be taxable in Singapore in YA 2009 as he would have been present in Singapore for periods exceeding the aggregate of 183 days in the first 12-month period commencing from 1 July 2008 (See Chart 2).
His income derived from the second engagement in 2009 will be taxable in Singapore in YA 2010 as he would be present in Singapore for periods exceeding the aggregate of 183 days in the second 12-month period ending 31 July 2009 (See Chart 2).
Chart 2: Taxable for YA 2009 and YA 2010 as physical presence in Singapore > 183 days in first 12-month period from 1 Jul 2008 to 30 Jun 2009 and second 12-month period from 1 Aug 2008 to 31 Jul 2009 respectively