The regulator is accountable to the Treasury and parliament and sets itself a target of responding to 80 per cent of letters from MPs within 15 working days, and 100 per cent within 20 working days.
But over the past year its response rate has suffered - an underperformance the City watchdog has attributed to the growing numbers of letters it received and the "particularly complex and sensitive" nature of their contents.
During 2018 the FCA received 294 letters from MPs and responded to 73.5 per cent of these within the 15-day timescale, but in 2019, when the regulator has so far received 405 letters, the response rate has dropped to 54.3 per cent.
The latest data, which was published this week as part of the FCA's service standards statistics for 2018-19, also showed the regulator's 20-day target decreased from 98 per cent in 2018 to 81.2 per cent in 2019.
The FCA said it has seen a "significant increase" in the volume of letters received this year, which has "adversely affected" its service targets.
The regulator said: "As well as the volume of letters received a number of them were particularly complex and sensitive, this required a lot of additional attention and internal and external input.
"We are working to ensure our processes are able to meet this increased demand, within service standards, whilst maintaining the quality and rigour of our responses."
The FCA also said it had received an "unusually high" number of requests under the Freedom of Information Act so far this year, a total of 743, which resulted in its response rate dropping from 94.4 per cent in 2018 to 88.5 per cent in 2019 - below its voluntary target of 90 per cent.
The City watchdog said additional resources were being recruited to improve its future overall compliance rate.
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