• Doubts are mounting over the Labour government's flagship growth and investment agenda, with one analyst warning further tax rises could be on the horizon.
• Finance Minister Rachel Reeves last week announced a series of reforms, including financial services deregulation and measures to boost pension investments.
• "These sort of regulatory changes ... if they don't get the economy moving, I think we're looking at more tax rises again," James Smith, economist at ING, told CNBC.

LONDON — Doubts are mounting over the Labour government's flagship growth and investment agenda, with one analyst warning further tax rises could be coming as soon as next year.

U.K. Finance Minister Rachel Reeves last week announced a series of reforms, including financial services deregulation and measures to boost pension investments — the latest in a slew of changes aimed at getting the country's economy growing again.

A higher economic growth rate could theoretically boost the government's tax take without the need to raise taxes further, because overall revenues would be higher. Labour has a fine balance to strike, however, in keeping taxes high enough to fund the country's depleted public services, while leaving businesses with enough cash to invest and grow.

"The Chancellor is walking a real tightrope with this one," James Smith, economist at ING, told CNBC's "Squawk Box Europe" on Friday.

"These sort of regulatory changes – not just in finance but in planning and other areas – if they don't get the economy moving, I think we're looking at more tax rises again," he said.

The Bank of England's former deputy governor, John Grieve, expressed doubt last week that the measures would spur growth, saying neither financial services deregulation nor pension reforms were "game changers."

"I think she [Reeves] is going to have to do some bigger things to try to boost private investment," Gieve told CNBC on Friday, citing planning and infrastructure projects as much more likely to boost the economy.

The reforms came just over two weeks after Reeves' bumper tax-and-spend budget, which included £40 billion ($51.8 billion) in tax hikes and changes to the country's debt rules — measures Reeves said were essential to rebalance the U.K.'s gaping deficit.

The independent Office for Budget Responsibility said at the time that the measures should drive the economy in the near-term, and raised its economic growth forecast by several percentage points over the next two years, while lowering it longer term. The OBR now expects U.K. real GDP growth of 1.1% in 2024, followed by expansion of 2% in 2025, before falling to 1.5%.

Businesses, however — which were especially hard hit by a sweeping increase in the National Insurance payroll tax — said Labour's plans were likely to curb hiring and discourage investment. On Monday, a consortium of major U.K. retailers wrote to Reeves warning that higher costs would be fed through to consumers as a result of the budget.

"The real risk for the chancellor — and for businesses as well — is that we get more of the same next year at the next budget if we don't see that response in growth coming through," ING's Smith said.

The Labour government did not immediately respond to CNBC's request for comment on further possible tax changes.

'Desperate' growth rates

The U.K. economy barely grew in the third quarter, eking out a less-than-expected 0.1% expansion, data from the Office for National Statistics showed Friday. Gross domestic product (GDP) fell by 0.1% in September, also below expectations and following growth of 0.2% the previous month

"This is desperate growth. We've had 1% growth, or around 1% growth now since the Financial Crisis. That's 15 years. So this is a well-established trend and we need to do something dramatic," Gieve said, commenting on the GDP data.

The third quarter was a time of significant uncertainty in the U.K., with the government accused of talking down the economy and spooking investors ahead of the Oct. 30 budget.

As such, some analysts argued the government's fiscal plans, and growth agenda more generally, should be given more time to bed in.

"Measuring success in the very short-term risks declaring the whole endeavour a failure before there's a chance for green shoots to reach the surface," Sarah Coles, head of personal finance at Hargreaves Lansdown, told CNBC via email Monday.

Paul Dales, chief U.K. economist at Capital Economics, said the plans were likely to be measured over the coming months and years in terms of how successfully economic growth holds up against the OBR forecasts — with any tax changes then set to follow suit.

"If it [growth] is weaker and that weakness is expected to be sustained, then it may mean taxes need to rise further to achieve forecasted levels of tax revenue," Dales said over email, noting that Capital Economics forecasts a sight uptick in growth. If there were further pressure to raise government spending, while all else stayed unchanged, higher taxes could be expected, he added.

Markets will now be watching whether the government's reforms can inject growth into the flagging British economy.

Nevertheless, Coles suggested that tax hikes — at least at the next fiscal statement in March — would be "highly unlikely."

"There's always the chance we could be hit by something out of the blue, which upends expectations, but at the moment Labour has committed to one major Budget a year, so anything substantial sooner would be a real surprise — especially after such a major fiscal event in October," Coles said.

"The coming months will give us a clearer picture of whether the government has got the balance right."