The letter, from all 14 local authorities, follows the annoucnement earlier this month of how the latest slice of government cash handed to councils to help with Covod-19 expenses will be distributed.
Lancashire County Council said that the £56m which it has been allocated so far goes less than half way to meeting its estimated coronavirus bill of £120m.
Blackpool Council said that it had been expecting a £21m grant from government, but has so far received just under £10m in the first two rounds of funding.
Lancashire's 12 district authorities saw their shares of the extra cash rocket from the tens of thousands to just below or just over £1m in most cases - but many of them warned that it still might not be enough to cover their costs.
The leaders say that Lancashire's overall share of the funding has reduced by £6million compared to the first tranche announced at the end of March- making the county one of the worst hit areas in the country in terms of the reduction between rounds one and two.
Work underatken by local authorities in the county includes establishing Community Support Hubs in each council area, contacting almost 30,000 of the most socially or clinically vulnerable residents to determine what support they need, sourcing millions of items of personal protective equipment to keep key workers safe and supporting the social care sector in the county.
Councillor Geoff Driver, leader of Lancashire County Council and chairman of the Lancashire Leaders group, said: " At the beginning of this crisis, the government called upon all local authorities to play their part in responding to the impact of the pandemic.
"Collectively, we have done magnificently in extremely difficult circumstances, and are continuing to do so. In fact, the Lancashire Resilience Forum, which all the county's local authorities support, is considered nationally as one of the most effective and has, in our view, provided decisive leadership in the early stages of the response to the outbreak.
"We are calling upon the government to deliver on the promises which have been made and ensure a full reimbursement of all costs, whether additional expenditure or loss of income, arising from the impact of Covid 19, and also acknowledgement that these costs will be incurred over a lengthy period of time.
"It is vital we have the necessary funding to support individuals, businesses and wider community-based organisations through this period as well as help them make a sustainable recovery from the impact of Covid-19.
"The inescapable fact is that local government as a whole needs a much bigger injection of funds from government to cover the financial burden caused by COVID-19 which includes lost income and planned savings not being achieved as well as the huge increases in expenditure."
Cllr Simon Blackburn, executive leader of Blackpool Council, desrbied the fundign allocations to date as a "great shock to an already strained system".
"The Secretary of State has publicly and repeatedly told councils to 'spend whatever needs to be spent' to tackle the Covid-19 crisis and we’ve kept his officials fully informed about how much we’ve spent thus far so to see our allocation cut from £6 million to £3.8 million left us flabbergasted.
"Officials in Whitehall keep reassuring us that council taxpayers won’t be left out of pocket at the end of all this, but I’m very nervous that we’ve so far had less than half of our spending repaid by Government.
"I will continue to seek reassurances about this from Ministers and Civil Servants, with the support of our MPs.”
Cllr Alyson Barnes, leader of Rossendale Borough Council, speaking on behalf distrct authorities, said: "Our local district councils have been at the forefront of the coronavirus response. We have managed to keep essential services going, protected the most vulnerable, supported our business and provided leadership for our areas. Our staff have risen to the challenge as key workers.
"Councils have already seen their budget hugely reduced over the last 10 years and we were in tough times financially even before the coronavirus. Responding to this unprecedented crisis is having a huge impact on our finances. On behalf of districts in Lancashire, I urge the government to fully reimburse local authorities in Lancashire for the cost dealing with the outbreak."
The communities secretary Robert Jenrick has previously said that councils will be given "the resources they need to meet [the] challenge" of dealing with the effects of Covid-19. He added last week that councls are currently receiving more additional funding than they say they need.
“If further resources are needed to meet the Covid-related costs that we have asked councils to bear, then obviously we will take that into consideration, " he told the daily Downing Street press briefing.
However, earlier this month, Mr. Jenrick gave evidence to a meeting of the Commons' housing, communities and local government select committee in which he warned that councils had been advised of the "specific tasks" which the government was asking them to carry out - including maintaining social care and housing rough sleepers - but that in relation to any other activity being undertaken "of their own volition" he "wouldn’t want anybody to labour under a false impression that what they are doing will be guaranteed funded by central government".