By bizn_fun 
The FCA has confirmed it received the letter and the BBC understands it will respond.
The reaction of financial markets has been closely watched in the run-up and aftermath of the Budget giving the impact the tax and spending policies could have on UK borrowing costs.
Many governments sell bonds – essentially IOUs – to raise money for public spending and in return they pay interest.
But how credible the markets deem a chancellor’s grip on the finances can affect how much it costs governments to borrow money.
Following Reeves’s Budget on Wednesday, the cost of government borrowing fell slightly, signalling a vote of confidence with the policy annoucements.
Reeves announced a series of tax rises, and extended a further three-year freeze of the thresholds at which people pay tax and higher income tax rates, meaning millions of people will be pulled in and have to pay more from their pay packets. She also scrapped the two-child benefit cap.
But the chancellor has faced since accusations she misled the public about the state of the public finances in the run-up.
Reeves repeatedly talked about a downgrade to the UK’s predicted economic productivity that would make it hard for her to meet her borrowing rules, fuelling speculation that the income tax rates themselves would be raised, which would break a manifesto pledge.
On 4 November, she used a rare pre-Budget speech in Downing Street to warn the UK’s productivity was weaker “than previously thought” and that “has consequences for the public finances too, in lower tax receipts.”
Then, on 10 November, Reeves told the BBC: “It would, of course be possible to stick with the manifesto commitments, but that would require things like deep cuts in capital spending.”
However, it has since emerged that the Office for Budget Responsbility (OBR), had told the Treasury on 31 October that it was on course to meet its main borrowing rule by £4.2bn, although the figure was less than the £9.9bn buffer Reeves had left herself last year.
In a letter to the Commons Treasury select committee, OBR chairman Richard Hughes revealed that he also told the chancellor on 17 September that the public finances were in better shape than widely thought.
As well as the Conservatives, the SNP has also written to the FCA urging it to look into claims of “deliberately false and misleading” briefings.
Reports in the run-up to the Budget had suggested the chancellor could have faced a £20bn gap in meeting her tax and spending rules as a result of the OBR’s productivity downgrade.