Groups representing police officers, shop and factory workers as well as transport and health staff say the plans would place men and women seeking compensation from an employer after an accident at a disadvantage.
Leading unions including Britain's biggest, Unite, have written to Holyrood's Justice Committee which is examining the proposals in the Courts Reform (Scotland) Bill.
At present, workers claiming more than £5000 can go to the Court of Session in Edinburgh to fight their case. They or the union acting for them can instruct a specialist advocate or counsel and if they win the lawyer's fee is paid, after the action is settled, by the other party.
Under the Bill only cases where the value of compensation exceeds £150,000 would be heard in the Court of Session, and smaller cases would go to the sheriff court or a new specialist personal injury court.
No automatic right to counsel exists in the lower courts, meaning either the claimant or his or her union has to pay for the advocate or rely on a solicitor who might not have the same specialist experience. It is likely insurance companies would continue hiring counsel - creating, the unions say, an unfair balance in the two parties' legal representation. Unite in its submission said: "Without the benefit of the specialist court and the right to employ counsel, victims of workplace accident and injury will face a significant uphill struggle in securing justice. Cases that ought to be won will be lost."
It added: "Unite will be left with an invidious choice to either leave our members fighting with one hand behind their back in terms of legal representation or find extra money in our already strained budgets. The truth is that the former is far more likely to be the only choice which we have."
Last year Unite, which has 200,000 members in Scotland, supported 800 cases of employees affected by working injury or illness in Scotland's courts.
Unions also believe because the reforms will make it harder for employees to pursue legal action, companies will pay less attention to hazards in the workplace, leading to more dangerous working conditions.
They are insisting the Bill should be amended to allow cases where an employee has been injured at work to be heard at the new specialist personal injury court and to have the automatic right to employ counsel.
As well as Unite, Unison, the National Union of Rail and Maritime Workers, the Scottish Police Federation, the Union of Shop, Distributive and Allied Workers, and the Associated Society of Locomotive Engineers and Firemen oppose the changes.
The Bill follows recommendations by Lord Gill to speed up the "justice system. But the Law Society of Scotland, which represents solicitors, warns the move could overwhelm the sheriff courts and lead to long delays for litigants.
The Association of British Insurers agrees with the value of cases being heard in the sheriff court being raised to £150,000, saying it would make "effective and efficient use of resources".
A Scottish Government spokeswoman said: "It is not true counsel will not be available in the sheriff court or the specialist personal injury court. It will continue to be the case that the sheriff will be able to approve the recovery of counsel's fees if considered appropriate.
"It is right the most complex cases should continue to benefit from such expertise and counsel's fees should be recoverable by the successful party in these circumstances. However, we do not agree that the cost of employing counsel should be recoverable in low-value, straightforward claims.
"We do not agree insurance companies habitually employ counsel to oppose low-value personal injury claims, as it would not be cost- effective to do so."