Markets now see more risk in the UK's 5-year bonds than those for the most heavily indebted eurozone members

• Yields on the five-year gilt soared more than 51 basis points on Monday to 4.579% amid fallout from the UK government's tax plan.
• Yields on five-year Greek and Italian bonds were 3.978% and 4.079%, respectively.

Borrowing costs over the medium term in the UK jumped on Monday as investors worry over a plan to cut taxes in Britain.

The yield on five-year bonds soared more than 51 basis points on Monday to 4.579%, overtaking the likes of Greece and Italy. Yields on five-year Greek and Italian bonds were 3.978% and 4.079%, respectively.

That indicates markets see more risk in the UK's medium-term gilt than in equivalent bonds for the most heavily indebted eurozone countries.

As of July, Greece had a debt-to-GDP ratio of 189.3% and Italy's was 152.6%, representing the top two in the currency union. The UK's was 99.6% in July.

British debt, along with the pound, tanked after the UK's new chancellor detailed a slew of prospective tax cuts and hinted more were coming. Investors feared the cuts would not only increase government debt but also fuel inflation, spurring the Bank of England to hike interest rates more aggressively and potentially drag the UK economy into a deep recession.

For now, yields on 10-year debt in the UK are still below those for both Greece and Italy, although the gap has begun to narrow.

But the five-year gilt yield's surge past similarly dated Greek and Italian bonds are also notable because the two southern European countries were among the flash points in the eurozone debt crisis a decade ago.

At the time, yields for the so-called PIIGS — Portugal, Italy, Ireland, Greece, and Spain — jumped as markets raised doubts about their ability to service their high levels of debt amid weak economic growth.