CT600 Box 405: Ring Fence Profits Explained
CT600 Box 405 reports ring fence profits — a special category of income that applies exclusively to companies extracting oil and natural gas on the UK Continental Shelf. For virtually all small and medium-sized companies in the UK, Box 405 will be zero and can be ignored entirely. This guide explains what ring fence profits are, who they affect, and what the ring fence tax regime involves.
What Is CT600 Box 405?
Box 405 appears in the tax calculation grid within Section 11 of the CT600. It sits alongside Box 400 (franked investment income) and Boxes 335–415, which collectively break down your company's taxable profits by category.
Box 405 specifically records profits that fall within the ring fence — a legally defined boundary that separates oil and gas extraction profits from a company's other trading activities. The ring fence concept exists to prevent oil and gas profits from being sheltered by losses or reliefs generated outside the energy sector.
What Is the Ring Fence in UK Tax Law?
In UK corporation tax law, ring fencing means that the profits and losses from a specified activity are treated as a separate pool, isolated from the company's other income and outgoings.
For oil and gas, the ring fence encompasses:
- Extraction of petroleum (crude oil and natural gas) from the UK Continental Shelf (UKCS)
- Rights to extract oil and gas
- Activities ancillary to extraction, such as processing crude at or near the extraction site
Who Uses Box 405?
Box 405 applies to companies carrying on ring fence trades — oil and gas extraction businesses operating on the UK Continental Shelf. In practice, this means:
- Major oil and gas companies operating North Sea platforms
- Companies holding petroleum extraction licences from the North Sea Transition Authority (formerly the Oil and Gas Authority)
- Businesses engaged in UK onshore oil extraction
- Trading companies
- Property companies
- Retailers, service businesses, consultancies
- Technology and software companies
- Any business that does not hold a petroleum extraction licence
Ring Fence Corporation Tax Rates
Ring fence profits are subject to a separate corporation tax rate, distinct from the main rates that apply to ordinary trading profits.
For ring fence trades:
- Ring fence corporation tax rate: 30% (reduced from 50% in 2015)
- Supplementary Charge: An additional levy on ring fence profits, currently 10% (reduced from 32% in 2016 and previously higher during the North Sea windfall discussions)
For context, the current standard corporation tax rates — 19% on profits up to £50,000, 25% on profits above £250,000, with marginal relief in between — apply to non-ring-fence trades only. If you are unsure which rates apply to your company, the UK corporation tax rates guide sets out the full rate structure for ordinary companies.
What Is the Supplementary Charge?
In addition to the ring fence corporation tax, ring fence companies pay a Supplementary Charge on Ring Fence Profits. This is reported separately on the CT600 and represents an extra layer of taxation on oil extraction profits above and beyond standard corporation tax.
The Supplementary Charge rate has varied significantly over the years, reflecting the political and economic sensitivity of North Sea taxation. It is currently 10%, having been reduced from 32% to encourage continued investment in the UKCS as production matures.
Ring fence companies must report their ring fence profits, ring fence corporation tax, and Supplementary Charge separately — using dedicated boxes on the CT600 — to allow HMRC to track this revenue stream distinctly.
How Ring Fence Losses Are Treated
A key feature of the ring fence regime is that non-ring-fence losses cannot be set against ring fence profits. This means a conglomerate company cannot use losses from a struggling retail division to shelter profits from its oil extraction subsidiary.
However, ring fence losses themselves are highly flexible:
- Ring fence losses can be carried back up to three years against ring fence profits (compared to one year for ordinary trading losses)
- Certain exploration and appraisal expenditure can be treated as a ring fence loss even before production begins
- The extended carry-back is designed to recognise the long investment cycle of oil exploration
Box 405 and the CT600 Computation
When Box 405 is completed, the ring fence profit figure is subjected to the ring fence corporation tax rate (30%) rather than the standard rate. The resulting tax is calculated separately from the tax on ordinary profits.
For companies with both ring fence and non-ring-fence activities, the CT600 computation must carefully separate the two, with different tax rates applied to each pool of profits. HMRC's guidance and software requirements reflect this complexity.
For a broader understanding of how the CT600 structures your tax computation — including how net chargeable profits are arrived at and how box values interrelate — see the CT600 Box 315 guide.
Why Box 405 Rarely Appears on Standard Returns
The North Sea oil and gas sector is dominated by large multinational companies. The capital requirements for offshore extraction, the regulatory framework (licences, environmental obligations, decommissioning liabilities), and the specialised tax regime mean that ring fence trades are simply not undertaken by typical small or medium-sized businesses.
There are a small number of UK onshore petroleum licences, but even these are tightly regulated and confined to specialist operators.
For the millions of small companies — consultancies, property businesses, e-commerce sellers, tradespeople, agencies — that file CT600 returns each year, Box 405 is irrelevant. It will not appear on a standard CT600 return, and CT600 preparation software for small companies will either leave it blank automatically or display it as zero.
Common Questions About Box 405
Do I need to complete Box 405 if my company produces or sells energy?
Not unless you extract oil or gas from the ground. Energy resellers, renewable energy generators, and companies that simply consume or trade in energy products are not ring fence companies. Box 405 is exclusively for petroleum extraction from the UK Continental Shelf or UK onshore fields.
My CT600 software shows Box 405 — should I enter anything?
No. Leave it blank or at zero. CT600 software often displays all boxes for completeness, but the vast majority of companies will have no ring fence profits to report.
What if I acquire an oil and gas business?
If your company acquires ring fence activities, you will need specialist oil and gas tax advice. The ring fence regime is complex, and the returns must be prepared by advisers with specific experience in UK upstream oil and gas taxation.
Summary
CT600 Box 405 captures ring fence profits from oil and gas extraction on the UK Continental Shelf. These profits are taxed at a 30% ring fence rate (plus a 10% Supplementary Charge) that is entirely separate from the standard corporation tax rates applying to ordinary businesses. For the vast majority of UK small companies, Box 405 is zero and can be safely ignored.
For a complete overview of how your company tax return is structured and what each section covers, the CT600 boxes explained guide provides a useful reference.